What is the purpose of the vertical guidelines?

[jcrickards (Julian Rickards)]

The horizontal guidelines (version 14) are identified but I can create vertical guidelines.

In some of the existing custom glyphs that I’m editing, the vertical guidelines are snapped to the left and right edges of the glyph, in others, the glyph is wider than the guidelines and in others, the glyph is much narrower than the space given by the vertical guidelines. In the main view of FontCreator, the vertical guidelines define the width of the white background.

Do these vertical guidelines define anything about the glyph such as space to the next character? Should the glyphs be centered on the 0 mark?

[Bhikkhu_Pesala]

Most glyphs like ABC will have a left and right side bearing, so the vertical guides will not be at the edge of the glyph. Maybe for glyphs like Underline _

Combining diacritical marks may be positioned left of the zero guide, and usually have zero advance with, so the right vertical line is also at zero.

Some glyphs, like Capital J, may have a negative left side bearing and a positive right side bearing.

Glyphs are not usually centred on the zero vertical line.