Attic Greek, like other dialects, was originally written in a local variant of the Greek alphabet. According to the classification of archaic Greek alphabets, which was introduced by Adolf Kirchhoff, the old-Attic system belongs to the “eastern” or “blue” type, as it uses the letters Ψ and Χ with their classical values (/ps/ and /kʰ/), unlike “western” or “red” alphabets, which used Χ for /ks/ and expressed /kʰ/ with Ψ. In other respects, Old Attic shares many features with the neighbouring Euboean alphabet (which is “western” in Kirchhoff’s classification). Like the latter, it used an L-shaped variant of lambda (Greek Lambda Athenian.svg) and an S-shaped variant of sigma (Greek Sigma Z-shaped.svg). It lacked the consonant symbols xi (Ξ) for /ks/ and psi (Ψ) for /ps/, expressing these sound combinations with ΧΣ and ΦΣ, respectively. Moreover, like most other mainland Greek dialects, Attic did not yet use omega (Ω) and eta (Η) for the long vowels /ɔː/ and /ɛː/. Instead, it expressed the vowel phonemes /o, oː, ɔː/ with the letter Ο (which corresponds with classical Ο, ΟΥ, Ω) and /e, eː, ɛː/ with the letter Ε (which corresponds with Ε, ΕΙ, and Η in later classical orthography). Moreover, the letter Η was used as heta, with the consonantal value of /h/ rather than the vocalic value of /ɛː/.
In the fifth century, Athenian writing gradually switched from this local system to the more widely used Ionic alphabet, native to the eastern Aegean Islands and Asia Minor. By the late fifth century, the concurrent use of elements of the Ionic system with the traditional local alphabet had become common in private writing, and in 403 BC, it was decreed that public writing would switch to the new Ionic orthography, as part of the reform following the Thirty Tyrants. This new system, also called the “Eucleidian” alphabet, after the name of the archon Eucleides, who oversaw the decision, was to become the Classical Greek alphabet throughout the Greek-speaking world. The classical works of Attic literature were subsequently handed down to posterity in the new Ionic spelling, and it is the classical orthography in which they are read today.