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DEFINITIONS OF TERMS USED IN THIS MANUAL

Typesetting Terms
Groff Terms
Mom Document Processing Terms

I use a number of typesetting-specific and groff-specific terms throughout this documentation, as well as a few terms that apply to mom herself. To make life easier, I'll explain them here. Refer back to this section should you encounter a word or concept you're not familiar with.

Typesetting terms
Ascender
The portion of a letter that extends above the bowl. For example, the letters a, c, and e have no ascenders. The letters b, d, and h do.
Baseline
The imaginary line on which the bottoms of capital letters and the bowls of lower case letters rest.
Ballot box
An unfilled square, usually cap-height in size, typically placed beside items in a checklist.
Bullet
A small, filled circle typically found beside items or points in a list.
Cap-height
The height of the tallest capital letter in a given font at the current point size.
Descender
The portion of a letter that extends beneath the baseline (j, q, y are letters with descenders).
Discretionary hyphen
A symbol inserted between two syllables of a word that indicates to a typesetting program the legal hyphenation points in the word. Normally, if hyphenation is turned on, groff knows where to hyphenate words. However, hyphenation being what it is (in English, at any rate), groff doesn't always get it right. Discretionary hyphens make sure it does. In the event that the word doesn't need to be hyphenated at all, groff leaves them alone. In groff, the discretionary hyphen is entered with 

	\%
(backslash followed by a percent).
Drop cap
A large, usually upper-case letter that introduces the first paragraph of a document or section thereof. The top of the drop cap usually lines up with the top of the first line of the paragraph, and typically "drops" several lines lower. Text adjacent to the drop cap is indented to the right of the letter until the bottom of the drop cap is reached, at which point text reverts to the left margin.
Em/en
An em is a relative measurement equal to the width of the letter M at a given point size in a given font. Since most Ms are designed square, an em is usually (but sometimes erroneously) considered to be the same size as the current point size (i.e. if the point size of the type is 12, one em equals 12 points). An en is equal to the width of a letter N (historically 2/3 of an em, although groff treats an en as 1/2 of an em). Typically, ems and ens are used to measure indents, or to define the length of dashes (long hyphens).
Family
The collective name by which a collection of fonts are known, e.g. Helvetica, Times Roman, Garamond.
Figure space/Digit space
A fixed width space that has the width of one digit. Used for aligning numerals in, say, columns or numbered lists. In groff, the figure space is entered with 

	\0
(backslash followed by a zero).
Fixed width space
Equal to word space, but does not expand or contract when text is justified. In groff, fixed width space is entered with 

	\<space>
where <space> means "hit the spacebar on your keyboard."
Font
The specific weight and shape of type within a family, e.g. light, medium, bold (which are weights), and roman, italic, condensed (which are shapes). By default, groff knows of four fonts within its default set of families: R (medium roman), I (medium italic), B (bold roman) and BI (bold italic).
Force justify
Sometimes, in justified text, a line needs to be broken short of the right margin. Force justifying means telling a typesetting program (like groff) that you want the line broken early AND that you want the line's word spacing stretched to force the line flush with the right margin.
Gutter
The vertical whitespace separating columns of type.
Justify/justification
Lines of type are justified when they're flush at both the left and right margins. Justification is the act of making both margins flush. Some people use the terms "left justified" and "right justified" to mean type where only the left (or right) margins align. I don't. See quad.
Kerning
Moving pairs of letters closer together to remove excess whitespace between them. In the days before phototypesetting, type was set from small, rectangular blocks of wood or metal, each block having exactly one letter. Because the edge of each block determined the edge of each letter, certain letter combinations (TA, for example) didn't fit together well and had to be mortised by hand to bring them visually closer. Modern typesetting systems usually take care of kerning automatically, but they're far from perfect. Professional typesetters still devote a lot of time to fitting letters and punctuation together properly.
Kern Units
A relative distance equal to 1/36 of the current point size. Used between individual letters for kerning. Different typesetting systems use different values (1/54 is popular), and sometimes call kern units by a different name.

Experts:
A kern unit has nothing to do with groff machine units.

Lead/leading
The distance from the baseline of one line of type to the line of type immediately beneath it. Pronounced "ledding." Also called line spacing. Usually measured in points.

In case you're interested... In previous centuries, lines of type were separated by thin strips of--you guessed it--lead. Lines of type that had no lead between them were said to be "set solid." Once you began separating them with strips of lead, they were said to be "leaded", and the spacing was expressed in terms of the number of points of lead. For this reason, "leading" and "line spacing" aren't, historically speaking, synonymous. If type was set 10 on 12, for example, the leading was 2 points, not 12. Nowadays, however, the two terms are used interchangeably to mean the distance from baseline to baseline.

Leaders
Single characters used to fill lines, usually to their end. So called because they "lead" the eye from one element of the page to another. For example, in the following (brief) Table of Contents, the periods (dots) are leaders. 

	Foreword............... 2
	Chapter 1.............. 5
	Chapter 2.............. 38
	Chapter 3.............. 60
Ligature
Ligatures are letters joined together to form a single character. The commonest are fi, fl, ff, ffi and ffl. Others are ae and oe. Occasionally, one sees an st ligature, but this is archaic and quite rare.
Picas/Points
There are twelve points in a pica, and six picas in an inch (hence 72 points to the inch). In the same way that gem-dealers have always used their own system of measurement for weight (carats), typographers have always used their own system of measurement for type.
Point Size
The nominal size of type, measured in points from the bottom of the longest descender to the top of the highest ascender. In reality, type is always fractionally smaller than its point size.
Quad
When only one margin of type is flush, lines of type are quadded in the direction of the flush margin. Therefore, quad left means the left margin is flush, the right isn't. Quad right means the right margin is flush, the left isn't. Quad centre means neither the left nor the right margin is flush; rather, lines of type are quadded on both sides so that type appears centred on the page.
Rag
Describes a margin that isn't flush. Rag right means the right margin isn't flush. Rag left means the left margin isn't flush. The expression "flush left/rag right" is sometimes used to describe type that is quadded left.
Shape
The degree of slant and/or the width of characters. (Technically speaking, this is not a proper typesetting term; however, it may help clarify some concepts presented in these documents.)

Some typical shapes are:

The term font, as it is used in these documents, refers to a combination of weight and shape.
Solid/set solid
When no lead is added between lines of type (i.e. the point size and linespacing are the same), the lines are said to be "set solid."
Track kerning/Line kerning
Sometimes, it's advantageous to increase or decrease the amount of space between every letter in a line by an equal (usually small) amount, in order to fit more (or fewer) characters on the line. The correct term is letter spacing, but track kerning and line kerning (and sometimes, just "kerning") have come to mean the same thing.
Unbreakable space
Equal to word space, however words separated by an unbreakable space will always be kept together on the same line. Expands and contracts like word space. Useful for proper names, which one should, whenever possible, avoid splitting onto two lines. In groff, unbreakable space is entered with 

	\~
(backslash followed by a tilde).
Weight
The thickness of the strokes of letterforms. Medium and Book have average thicknesses and are the weights used for most of the text in books, magazines, newspapers, etc. Light has strokes slightly thinner than Medium or Book, but is still acceptable for most text. Semibold, Bold, Heavy and Black all have strokes of increasing thickness, making them suitable for heads, subheads, headlines and the like.
Word space
The amount of whitespace between words. When text is justified, word space expands or contracts to make the margins flush.
x-height
The height of a lower case letter x in a given font at a given point size. Generally used to mean the average height of the bowl of lower case letters.

Groff terms
Alias
A macro invoked by a name different from its "official" name. For example, the official name of the macro to change family is FAMILY. Its alias is FAM. Aliases may be created for any macro (via the ALIAS macro) provided the alias uses a name not already taken by the mom macros or one of the groff primitives. For a complete list of words or names you must not use, see the list of reserved words.
Arguments
Parameters or information needed by a macro to do its job. For example, in the macro 

	.PT_SIZE 12
"12" is the argument. In the macro 

	.QUAD LEFT
LEFT is the argument. Arguments are separated from macros by spaces. Some macros require several arguments; each is separated by a space.
Comment Lines
Input lines introduced with the comment character 

	\#
When processing output, groff silently ignores everything on a line that begins with the comment character.
Control Lines
Instructions to groff that appear on a line by themselves, which means that "control lines" are either macros or groff primitives. Control lines begin with a period or, occasionally, an apostrophe.
Filled lines/fill mode
Automatic justification or quadding. In fill mode, the ends of lines as they appear in your text editor are ignored. Instead, words from adjoining input lines are added one at a time to the output line until no more words fit. Then, depending whether text is to be justified or quadded (left, right, or centre), and depending on whether automatic hyphenation is turned on, groff attempts to hyphenate the last word, or, barring that, spreads and breaks the line (when justification is turned on) or breaks and quads the line (when quadding is turned on).

Nofill mode (non-filled text) means that groff respects the ends of lines as they appear in your text editor.

Inline escapes
Instructions issued to groff that appear as part of an input line (as opposed to macros, which must appear on a line by themselves). Inline escapes are always introduced by the backslash character. For example, 

	A line of text with the word T\*[BU 2]oronto in it
contains the inline escape \*[BU 2] (which means "move the letter 'o' 2 kern units closer to the letter 'T'").

Mom's inline escapes always take the form \*[ESCAPE], where ESCAPE is composed of capital letters, sometimes followed immediately by a digit, sometimes followed by a space and a numeric argument. Groff's escapes begin with the backslash character but typically have no star and are in lower case. For example, the mom escapes to move forward 6 points on a line are either 

	\*[FP6]  or  \*[FWD 6p]
while the groff escape for the same thing is 

	\h'6p'
Input line
A line of text as it appears in your text editor.
Macros
Instructions embedded in a document that determine how groff processes the text for output. mom's macros always begin with a period, on a line by themselves, and must be typed in capital letters. Typically, macros contain complex commands issued to groff--behind the scenes--via groff primitives.
Machine units
A machine unit is 1/1000 of a point when the groff device is ps. ("ps" means "PostScript"--the default device for which groff prepares output, and the device for which mom was specifically designed.)
Numeric argument
An argument that has the form of a digit. Numeric arguments can be built out of arithmetic expressions using +, -, *, and / for plus, minus, times, and divided-by respectively. If a numeric argument requires a unit of measure, a unit of measure must be appended to every digit in the argument. For example: 

	.ALD 1i-1v
NOTE: groff does not respect the order of operations, but rather evaluates arithmetic expressions from left to right. Parentheses must be used to circumvent this peculiarity. Not to worry, though. The likelihood of more than just the occasional plus or minus sign when using mom's macros is slim.
Output line
A line of text as it appears in output copy.
Primitives
The two-letter, lower case instructions groff uses as its native command language, and out of which macros are built.
String Argument
Technically, any argument that is not numeric. In this documentation, string argument means an argument that requires the user to input text. For example, in the macro 

	.TITLE "My Pulitzer Novel"
"My Pulitzer Novel" is a string argument.

Because string arguments must be enclosed by double-quotes, you can't use double-quotes as part of the string argument. If you need double-quotes to be part of a string argument, use the inline escapes \(lq and \(rq (leftquote and rightquote respectively) in place of the double-quote character (").

Unit of measure
The single letter after a numeric argument that tells mom what measurement scale the argument should use. Common valid units are:

i = inches
p = points
P = picas
c = centimetres
m = ems
n = ens
v = the current leading (line space)

Units of measure must come immediately after the numeric argument (i.e. with no space between the argument and the unit of measure), like this: 

	.ALD 2v
	.LL  39P
	.IL  1i
The above example advances 2 line spaces and sets the line length to 39 picas with a left indent of 1 inch.

IMPORTANT: Most mom macros that set the size or measure of something MUST be given a unit of measure. mom's macros do not have default units of measure. There are a couple of exceptions, the most notable of which are PT_SIZE and LS. Both use points as the default unit of measure, which means you don't have to append "p" to their argument.

You can enter decimal values for any unit of measure. Different units may be combined by adding them together (e.g. 1.5i+2m, which gives a measure of 1-1/2 inches plus 2 ems).

NOTE: a pica is composed of 12 points, therefore 12.5 picas is 12 picas and 6 points, not 12 picas and 5 points. If you want 12 picas and 5 points, you have to enter the measure as 12P+5p.

Zero-width character
The inline escape that allows you to print a literal period, apostrophe and, if output lines are filled, a space that falls at the beginning of an input line. It looks like this: 

	\&
(backslash followed by an ampersand).

Normally, groff interprets a period (or an apostrophe) at the beginning of an input line as meaning that what follows is a control line. In fill modes, groff treats a space at the beginning of an input line as meaning "start a new line and put a space at the beginning of it." If you want groff to interpret periods and apostrophes at the beginning of input lines literally (i.e. print them), or spaces at the beginning of input lines as just garden variety word spaces, you must start the line with the zero-width character.

Mom's Document Processing Terms
Blockquote
Cited material other than quotes. Typically set at a smaller point size than paragraph text, indented from the left and right margins. Blockquotes are filled.
Control macro
Macros used in document processing to control/alter the appearance of document elements (e.g. heads, quotes, footnotes, headers, etc.).
Document header/docheader
Document information (title, subtitle, author, etc) output at the top of page one.
Epigraph
A short, usually cited passage that appears at the beginning of a chapter, story, or other document.
Footer/page footer
Document information (frequently author and title) output in the bottom margin of pages after page one. Not to be confused with footnotes, which are considered part of running text.
Head
A title that introduces a major section of a document.
Header/page header
Document information (frequently author and title) output in the top margin of pages after page one.

NOTE: In terms of content and style, headers and footers are the same; they differ only in their placement on the page. In most places in this documentation, references to the content or style of headers applies equally to footers.

Linebreak/author linebreak
A horizontal gap in running text, frequently set off by typographic symbols such as asterisks or daggers. Used to indicate a shift in the content of a document (e.g. a scene change in a short story). Also commonly called a scene break or a section break.
Paragraph head
A title joined to the body of a paragraph; hierarchically one level beneath subheads.
Quote
A quote, to mom, is a line-for-line setting of quoted material (e.g. poetry, song lyrics, or a snippet of programming code). You don't have to use BR with quotes.
Running text
In a document formatted with mom, running text means text that forms the body of the document, including elements such as heads and subheads. Docheaders, headers, footers and page numbers are NOT part of running text.
Subhead
A title used to introduce secondary sections of a document; hierarchically one level beneath sections introduced by heads.
Toggle
A macro or tag that, when invoked without an argument, begins something or turns a feature on, and, when invoked with ANY argument, ends something or turns a feature off. See Example 3 of the section How to read macro arguments.

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