Contextual substitutions can be controlled by enabling or disabling the composition options of a TrueType GX font in WorldText on the Mac OS 9 CD or in TextEdit in Mac OS X. Fonts commonly have features called "common ligatures" (such as the "fl" example), "rare ligatures" (such as inscriptional ME and MD ligatures), "archaic non-terminal s" (for automatically substituting the letter "s" with the archaic form that looked more like an "f", except at the ends of words), and even choices between entirely separate sets of glyph designs, such as more and less ornate forms.