The iota subscript is a diacritic mark in the Greek alphabet shaped like a small vertical stroke or miniature iota 〈ι〉 placed below the letter. It can occur with the vowel letters eta 〈η〉, omega 〈ω〉 and alpha 〈α〉. It represents the former presence of an [i] offglide after the vowel, forming a so-called "long diphthong". Such diphthongs, ηι, ωι, αι (ᾱι), phonologically distinct from the corresponding normal ("short") diphthongs ει, οι, αι (ᾰι), were a feature of ancient Greek in the classical era. The offglide was lost in pronunciation during the Hellenistic period, with the result that from approximately the 1st century B.C. onwards the former diphthongs were no longer distinguished from the simple long vowels η, ω, ᾱ respectively. During the Roman and Byzantine eras, the 〈ι〉, now mute, was sometimes still written as a normal letter, but often simply left out. The iota subscript was invented as an editorial symbol marking the places where such spelling variation occurred by Byzantine philologists in the 12th century AD. 
The alternative practice, of writing the mute iota as a full-sized letter 〈ι〉, is known as iota adscript.
In Greek, the subscript is called ὑπογεγραμμένη (hypogegramménē, the perfect participle form of the verb ὑπογράφω, hypográphō, 'write below'). Analogously, the adscript is sometimes called προσγεγραμμένη, from the verb προσγράφω, 'to write before'.
The Greek names are grammatically feminine participle forms because in medieval Greek the name of the letter "iota", to which they implicitly refer, was sometimes construed as a feminine noun (unlike in classical and in modern Greek, where it is neuter.) The Greek terms, transcribed as ypogegrammeni and prosgegrammeni respectively, were also chosen for use in character names in the computer encoding standard Unicode.
As a phonological phenomenon, the original diphthongs denoted by 〈ᾳ, ῃ, ῳ〉 were traditionally called "improper diphthongs" in older English works. More recent works prefer the term "long diphthongs".
The iota subscript occurs most frequently in certain inflectional affixes of ancient Greek, especially in the dative endings of many nominal forms (e.g. τῷ ἀνθρώπῳ, τῇ πολιτίᾳ, τῇ γλώσσῃ) as well as in certain verb forms of the subjunctive mood (e.g. λύσῃς, -ῃ). Besides these it also occurs in the roots of certain words and names, for instance ᾠδή 'ode' (and its derivatives: ᾠδείον 'odeion'; τραγῳδία 'tragedy' etc.); ᾅδης 'Hades'; θρᾴκη 'Thrace'.
The iota subscript is today considered an obligatory feature in the spelling of Ancient Greek, but its usage is subject to some variation. In some modern editions of classical texts, the historical spelling with a restored full-size adscript 〈ι〉 is preferred. The same is generally true for works dealing with epigraphy, paleography or other philological contexts where adherence to original historical spellings is considered important.
Different conventions exist for the treatment of subscript/adscript iota with uppercase letters. In Western printing, the most common practice is to use subscript diacritics only in lowercase environments and to use an adscript (i.e. a normal full-sized iota glyph) instead whenever the host letter is capitalized. When this happens in a mixed-case spelling environment (i.e. with only the first letter of a word capitalized, as in proper names and at the beginning of a sentence), then the adscript iota regularly takes the shape of the normal lowercase iota letter (e.g. ᾠδεῖον → Ὠιδεῖον). In an all-capitals environment, the adscript is also regularly capitalized (ΩΙΔΕΙΟΝ). In Greece, a more common convention is to print subscript diacritics both with lowercase and uppercase letters alike. Yet another, intermediate convention is to use lowercase adscript iotas both for mixed-case and for all-capitals words (e.g. ΩιΔΕΙΟΝ), or to use a special glyph in the shape of a smaller capital iota in the latter case (ΩΙΔΕΙΟΝ).
In Modern Greek, subscript iota was generally retained in use in the spelling of the archaizing Katharevousa. It can also be found regularly in older printed Demotic in the 19th and early 20th century, but it is often absent from the modern spelling of present-day Standard Modern Greek. Even when present-day Greek is spelled in the traditional polytonic system, the number of instances where a subscript could be written is much smaller than in older forms of the language, because most of its typical grammatical environments no longer occur: the old dative case is not used in Modern Greek except in a few fossilized phrases (e.g. "ἐν τῷ μεταξύ" "in the meantime"; "δόξα τῷ θεῷ" "thank God!"), and the old spellings with -ῃς/ῃ in subjunctive verbs have been analogically replaced by those of the indicatives with -εις/-ει (e.g. θα γράφῃς → θα γράφεις). In the monotonic standard orthography, subscript iota is not used.
- ζῷο-ν "animal" + -λογίᾱ → New Latin zōo-logia (not zōio-logia) → English zoology.
In a few Latin words that were borrowed from Greek at an early date, an old long diphthong is reflected as 〈œ〉, e.g. τραγῳδία → tragœdia, "tragedy".
Computer encoding 
In the Unicode standard, iota subscript is represented by a non-spacing combining diacritic character U+0345 "Combining Greek Ypogegrammeni". There is also a spacing clone of this character (U+037A, "ͺ"), as well as 36 precomposed characters, representing each of the usual combinations of iota subscript with lowercase α, η and ω, with and without any of the accent and breathing diacritics. In addition, for capitalized ("titlecase") use, Unicode provides a corresponding set of 27 precomposed codepoints with "prosgegrammeni" (ᾳ → ᾼ). Despite their name, which implies the use of an adscript glyph, these codepoints are defined as being equivalent to a combination of the base letter and the combining subscript character U+0345, just like their lowercase equivalents. They may be variously realized with either a subscript diacritic or a full-sized adscript iota glyph, depending on the font design. For use in all-capitals ("uppercase"), Unicode additionally stipulates a special case-mapping rule according to which lowercase letters should be mapped to combinations of the uppercase letter and uppercase iota (ᾳ → ΑΙ).
- Woodard, Roger D. (2008). "Attic Greek". The ancient languages of Europe. Cambridge: Cambridge University Press. p. 19.
- McLean, Bradley H. (2011). New Testament Greek: an introduction. Cambridge: Cambridge University Press. p. 20.
- Metzger, Bruce Manning (1981). Manuscripts of the Greek bible: an introduction to Greek palaeography. Oxford: Oxford University Press. p. 28.
- Sihler, Andrew L. (2008). New comparative grammar of Greek and Latin. Oxford: Oxford University Press. p. 59.
- Dickey, Eleanor (2007). Ancient Greek scholarship. Oxford: Oxford University Press. p. 256.
- Babiniotis, Georgios. "προσγράφω". Lexiko tis Neas Ellinikis Glossas.
- Babiniotis, Georgios. "υπογράφω". Lexiko tis Neas Ellinikis Glossas.
- e.g. Smyth, Herbert W. (1920). Greek grammar for colleges. New York: American Book Company. p. 9.; von Ostermann, George Frederick; Giegengack, Augustus E. (1936). Manual of foreign languages for the use of printers and translators. United States. Government Printing Office. p. 81.
- e.g. Mastronarde, Donald J (1993). Introduction to Attic Greek. Berkely: University of California Press. p. 9f.
- Nicholas, Nick. "Titlecase and Adscripts". Retrieved 5 August 2012.
- The difference in number between uppercase and lowercase precomposed characters is due to the fact that there are no uppercase combinations with only an accent but no breathing mark, because such combinations do not occur in normal Greek orthography (uppercase letters with accents are used only word-initially, and word-initial vowel letters always have a breathing mark).
- Thesaurus Linguae Graecae. "The beta code manual". Retrieved 5 August 2012.