OpenType fonts can have OpenType features, like small caps for instance; When enabled, the feature substitutes the selected characters with their small cap equivalent glyphs contained in that font. There are lots of these features (swash alternates, oldstyle figures, tabular figures, fractions, super- and subscripts, standard and discretionary ligatures, contextual alternates, titling alternates, petite caps, and the list goes on and on). Of course not all OpenType fonts support all features; Some have no features. Some have only a few glyphs, or even no glyphs at all.
Below you can see FF Yoga’s small caps; Arno setting a swash cap followed by standard and discretionary ligatures; FF Unit’s alternate a and g accessible via stylistic sets 1 and 2; Ingeborg’s default (ranging) figures and lining figures; Medusa’s out of control swash alternates; Amster’s case feature. This is one of the last features that graphic designers discover. It changes all selected characters to uppercase, and shifts the punctuation up (or replaces it completely) to better suit the cap forms. When reviewers write about these features in typefaces, they’re often called case-sensitive punctuation. Here you see Amster has parentheses drawn specifically for all-caps settings; Two more: Acta Display sets arbitrary fractions, like 7/10; and so does Scotch Modern which can also produce these stacked “nut” fractions.
TrueType fonts don’t have these kinds of features, so if there is a set of small caps or oldstyle figures or whatever, these come as a separate font alongside the regular style they extend. This is also true of PostScript Type 1 fonts, and the additional fonts containing alternate glyphs are called expert sets, since it takes an expert-level typographer to know what they are and how to use them, apparently.
Also, most popular webfont formats are based on OpenType, so you can use OpenType features in webfonts too. Here’s a showcase of OpenType features at work on the web from FontFont.
This is a direct advantage to the supply side (the type designer), and an indirect one to the demand side (the graphic designer who uses type). Most type designers prefer drawing and editing PostScript outlines, so the majority of fonts currently being released to the design market are OpenType fonts. TrueType fonts require TrueType outlines, which differ from PostScript outlines in the way the math that’s used describes curves, and in some other technical ways. In terms of which format a commercial printer more readily accepts for a press job, there still seems to be a bias toward OpenType fonts with PostScript outlines, probably because of past problems caused by low-quality free fonts in documents, or the requirements of some decades-old RIP (raster image processor) technologies.
It’s true that OpenType fonts can support tens of thousands of glyphs, many more than the limit of 256 set in PostScript Type 1. TrueType fonts are built on the same standard that OpenType is in terms of glyph capacity, though many older TrueType fonts contain exactly 256 characters due to the constraints of earlier sources.
Remember how above I said that OpenType fonts can have PostScript outlines? Well they can also have TrueType outlines. These fonts are usually labeled OT TTF or OTTT, for OpenType with TrueType outlines and have a .ttf extension (though technically they can also have an .otf extension). Most OpenType fonts however have PostScript outlines, recorded in the Compact Font Format, or CFF and have an .otf extension. What does this mean? Well, it means you can’t tell what kind of font you’ve got just by looking at the file extension, but if it’s got OpenType features, it’s an OpenType font, whether its outlines are PostScript or TrueType.
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