[5] In Archaic and early Classical times, the Greek alphabet existed in many local variants, but, by the end of the 4th century BC, the Ionic-based Euclidean alphabet, with 24 letters, ordered from alpha to omega, had become standard throughout the Greek-speaking world[6] and is the version that is still used for Greek writing today.

[9] By the 9th century, Byzantine scribes had begun to employ the lowercase form, which they derived from the cursive styles of the uppercase letters.

For a number of letters, sound values differ considerably between Ancient and Modern Greek, because their pronunciation has followed a set of systematic phonological shifts that affected the language in its post-classical stages.

These signs were originally designed to mark different forms of the phonological pitch accent in Ancient Greek.

[28] Aristophanes of Byzantium also was the first to divide poems into lines, rather than writing them like prose, and also introduced a series of signs for textual criticism.

Although it is not a diacritic, the comma has a similar function as a silent letter in a handful of Greek words, principally distinguishing ό,τι (ó,ti, 'whatever') from ότι (óti, 'that').

[34] In modern scholarly transliteration of Ancient Greek, ⟨κ⟩ will usually be rendered as ⟨k⟩, and the vowel combinations ⟨αι, οι, ει, ου⟩ as ⟨ai, oi, ei, ou⟩.

Five were reassigned to denote vowel sounds: the glide consonants /j/ (yodh) and /w/ (waw) were used for [i] (Ι, iota) and [u] (Υ, upsilon); the glottal stop consonant /ʔ/ (aleph) was used for [a] (Α, alpha); the pharyngeal /ʕ/ (ʿayin) was turned into [o] (Ο, omicron); and the letter for /h/ (he) was turned into [e] (Ε, epsilon).

Greek was originally written predominantly from right to left, just like Phoenician, but scribes could freely alternate between directions.

There were initially numerous local (epichoric) variants of the Greek alphabet, which differed in the use and non-use of the additional vowel and consonant symbols and several other features.

[50] Athens used a local form of the "light blue" alphabet type until the end of the 5th century BC, which lacked the letters Ξ and Ψ as well as the vowel symbols Η and Ω.

The name of lambda is attested in early sources as λάβδα besides λάμβδα;[55][15] in Modern Greek the spelling is often λάμδα, reflecting pronunciation.

[15] Similarly, iota is sometimes spelled γιώτα in Modern Greek ([ʝ] is conventionally transcribed ⟨γ{ι,η,υ,ει,οι}⟩ word-initially and intervocalically before back vowels and /a/).

In the following group of consonant letters, the older forms of the names in Ancient Greek were spelled with -εῖ, indicating an original pronunciation with -ē.

[56] Like Latin and other alphabetic scripts, Greek originally had only a single form of each letter, without a distinction between uppercase and lowercase.

This distinction is an innovation of the modern era, drawing on different lines of development of the letter shapes in earlier handwriting.

Besides the upright, straight inscriptional forms (capitals) found in stone carvings or incised pottery, more fluent writing styles adapted for handwriting on soft materials were also developed during antiquity.

Ancient handwriting developed two distinct styles: uncial writing, with carefully drawn, rounded block letters of about equal size, used as a book hand for carefully produced literary and religious manuscripts, and cursive writing, used for everyday purposes.

In the 9th and 10th centuries, uncial book hands were replaced with a new, more compact writing style, with letter forms partly adapted from the earlier cursive.

[78][79] In May 2021 the World Health Organization announced that the variants of SARS-CoV-2 of the virus would be named using letters of the Greek alphabet to avoid stigma and simplify communications for non-scientific audiences.

The glyph shapes used for these letters in specialized phonetic fonts is sometimes slightly different from the conventional shapes in Greek typography proper, with glyphs typically being more upright and using serifs, to make them conform more with the typographical character of other, Latin-based letters in the phonetic alphabet.

Nevertheless, in the Unicode encoding standard, the following three phonetic symbols are considered the same characters as the corresponding Greek letters proper:[83] On the other hand, the following phonetic letters have Unicode representations separate from their Greek alphabetic use, either because their conventional typographic shape is too different from the original, or because they also have secondary uses as regular alphabetic characters in some Latin-based alphabets, including separate Latin uppercase letters distinct from the Greek ones.

The symbol in Americanist phonetic notation for the voiceless alveolar lateral fricative is the Greek letter lambda ⟨λ⟩, but ⟨ɬ⟩ in the IPA.

The IPA symbol for the palatal lateral approximant is ⟨ʎ⟩, which looks similar to lambda, but is actually an inverted lowercase y. Greek letters were also used to write numbers.

This system has remained in use in Greek up to the present day, although today it is only employed for limited purposes such as enumerating chapters in a book, similar to the way Roman numerals are used in English.

[84] This naming tradition was initiated by the foundation of the Phi Beta Kappa society at the College of William and Mary in 1776.

The Roman alphabet has a similar extended form with such double-digit letters when necessary, but it is used for columns in a table or chart rather than chapters of an organization.

While their use in normal typography of Greek is purely a matter of font styles, some such variants have been given separate encodings in Unicode.

Unicode supports polytonic orthography well enough for ordinary continuous text in modern and ancient Greek, and even many archaic forms for epigraphy.

Combining and spacing (letter-free) diacritical marks pertaining to Greek language: IBM code pages 437, 860, 861, 862, 863, and 865 contain the letters ΓΘΣΦΩαδεπστφ (plus β as an alternative interpretation for ß).