Eta's presence in the Greek alphabet used today is itself a piece of script history distinct from the letter's ancient sound and number, covered elsewhere in this atlas. Archaic Athens used its own local, or epichoric, alphabet, which had no separate sign for a long e sound and simply wrote both long and short e with epsilon; the Ionic alphabet of Miletus, by contrast, had already repurposed an old consonant letter, eta, originally standing for the sound h in many other Greek dialects, to mark the long vowel instead. When Athens formally adopted the Ionic alphabet by decree in 403 BCE, under the archon Eucleides, eta's consonant value was dropped in Athenian and most subsequent Greek entirely, and the letter became purely the long-e vowel still used in modern Greek; the h sound it once carried survives today only in the rough breathing mark placed over vowels in polytonic Greek spelling.
Modern technical use of eta is comparatively minor: physicists and engineers use lowercase eta for efficiency ratios and for the viscosity of a fluid, and astronomers, following the Bayer star-naming convention of 1603, use Eta to mark a constellation's seventh-brightest star. The letter's two-stage history, an old consonant repurposed as a vowel through a specific, dated political decision, has no real parallel among the Latin letters covered elsewhere in this atlas, whose shape changes were generally gradual rather than decreed.