As a part of the type design process, type designers assign kerning values (or ‘pairs’) to each possible combination of characters / glyphs in a typeface set. As you can imagine, this is a huge undertaking and is one of the more time consuming parts of designing a typeface (you might notice with many free fonts that the kerning pairs are pretty awful—usually because the kerning values have been set to default in the typeface creation application). Metrics refers to the kerning pairs assigned by the type designer. For well-designed typefaces, this is a good setting to use for most text. Optical kerning adjusts the spacing between adjacent characters based on their shapes. Some fonts include robust (metrics) kern-pair specifications. However, when a font includes only minimal built-in kerning or none at all, or if you use two different typefaces or sizes in one or more words on a line, you may want to use the optical kerning option. Type tip: The differences between metrics and optical kerning values can be pretty subtle for many typefaces. It’s best to see what each option does to your type before committing one way or the other.