In its classical and modern forms, the alphabet has 24 letters, ordered from alpha to omega. Like Latin and Cyrillic, Greek originally had only a single form of each letter; it developed the letter case distinction between upper-case and lower-case forms in parallel with Latin during the modern era.
Sound values and conventional transcriptions for some of the letters differ between Ancient Greek and Modern Greek usage, because the pronunciation of Greek has changed significantly between the 5th century BC and today. Modern and Ancient Greek use different diacritics. The traditional orthography, which is used for Ancient Greek and sometimes for Modern Greek, has many diacritics, such as accent marks for pitch accent ("polytonic"), the breathing marks for the presence and absence of the initial /h/ sound, and the iota subscript for the final historical /i/ sound. In standard Modern Greek spelling, orthography has been simplified to a monotonic system, which uses only two diacritics: the acute accent and diaeresis.
In both Ancient and Modern Greek, the letters of the Greek alphabet have fairly stable and consistent symbol-to-sound mappings, making pronunciation of words largely predictable. Ancient Greek spelling was generally near-phonemic. 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.
Among consonant letters, all letters that denoted voiced plosive consonants (/b, d, g/) and aspirated plosives (/pʰ, tʰ, kʰ/) in Ancient Greek stand for corresponding fricative sounds in Modern Greek. The correspondences are as follows:
Among the vowel symbols, Modern Greek sound values reflect the radical simplification of the vowel system of post-classical Greek, merging multiple formerly distinct vowel phonemes into a much smaller number. This leads to several groups of vowel letters denoting identical sounds today. Modern Greek orthography remains true to the historical spellings in most of these cases. As a consequence, the spellings of words in Modern Greek are often not predictable from the pronunciation alone, while the reverse mapping, from spelling to pronunciation, is usually regular and predictable.