Horizontal glyph metrics»
Horizontal glyph metrics (usually referred to as glyph metrics or just metrics) are metric values of individual glyphs that are used to compute line lengths. This includes advance widths and sidebearings.
Advance width is the white space taken by a glyph, regardless of the marking portions of the glyph. For example, the space character has an advance width, even though it has no outline. It is not unusual to give numerals the same advance width, even though the space taken up by their glyph outlines may vary. It is possible for a marking glyph to have no advance width at all, as is sometimes done with combining accent marks.
Sidebearings are the amount of white space on each side of a glyph. The distance between the origin of the glyph and the first marking point on the glyph outline is the left sidebearing. The distance between the farthest right marking portion of the glyph and the advance width is the right sidebearing. When the glyph takes up less space than the advance width, inside the sidebearings, that creates positive sidebearings. Sometimes part of a glyph can “stick out” beyond the advance width; this creates a negative sidebearing. For example, if the right side of the letter “f” goes beyond the advance width, it has a negative sidebearing. An accent mark with no advance width might be centered on the X=0 origin line, and have both sidebearings negative.
Abbreviations RSB (right sidebearing) and LSB (left sidebearing) are sometimes used in FontLab.
Vertical glyph metrics»
When typing text in some Asian languages, it is often necessary to specify the vertical alignment of characters in the text. In this case, information about the vertical glyph metrics is stored in the font file.
Usually, within a single font, all Chinese, Japanese or Korean glyphs used in vertical layout have the same vertical advance “width” (called the vertical advance vector) so only the position of the glyph within the rectangular glyph cell needs to be specified.
However, it is possible to adjust the vertical advance vector of individual glyphs.
Note that this information is only used by applications that support vertical text layout, and only if the vertical glyph metric information is specified for all glyphs. Do not confuse vertical glyph metrics (specific to each glyph in fonts intended for vertical writing) with vertical font metrics. Vertical font metrics are font-wide, such as ascender and descender, and are used in text that is set horizontally.