Greek orthography has used a variety of diacritics starting in the Hellenistic period. The more complex polytonic orthography (), which includes five diacritics, notates Ancient Greek phonology. The simpler monotonic orthography (), introduced in 1982, corresponds to Modern Greek phonology, and requires only two diacritics.
Polytonic orthography () is the standard system for Ancient Greek and Medieval Greek. The acute accent (´), the circumflex (ˆ), and the grave accent (`) indicate different kinds of pitch accent. The rough breathing (῾) indicates the presence of the sound before a letter, while the smooth breathing (᾿) indicates the absence of .
Since in Modern Greek the pitch accent has been replaced by a dynamic accent (stress), and was lost, most polytonic diacritics have no phonetic significance, and merely reveal the underlying Ancient Greek etymology.
Monotonic orthography () is the standard system for Modern Greek. It retains two diacritics: a single accent or tonos (΄) that indicates stress, and the diaeresis ( ¨ ), which usually indicates a hiatus but occasionally indicates a diphthong: compare modern Greek (, "lamb chops"), with a diphthong, and (, "little children") with a simple vowel. A tonos and a diaeresis can be combined on a single vowel to indicate a stressed vowel after a hiatus, as in the verb (, "I feed").
Although it is not a diacritic, the hypodiastole (comma) has in a similar way the function of a sound-changing diacritic in a handful of Greek words, principally distinguishing (, "whatever") from (, "that").
History
The original Greek alphabet did not have diacritics. The Greek alphabet is attested since the 8th century BC, and until 403 BC, variations of the Greek alphabet—which exclusively used what are now known as capitals—were used in different cities and areas. From 403 on, the Athenians decided to employ a version of the Ionian alphabet. With the spread of Koine Greek, a continuation of the Attic dialect, the Ionic alphabet superseded the other alphabets, known as epichoric, with varying degrees of speed. The Ionian alphabet, however, also consisted only of capitals.
Introduction of breathings
The rough and smooth breathings were introduced in classical times in order to represent the presence or absence of an in Attic Greek, which had adopted a form of the alphabet in which the letter ⟨Η⟩ (eta) was no longer available for this purpose as it was used to represent the long vowel .
Introduction of accents
During the Hellenistic period (3rd century BC), Aristophanes of Byzantium introduced the breathings—marks of aspiration (the aspiration however being already noted on certain inscriptions, not by means of diacritics but by regular letters or modified letters)—and the accents, of which the use started to spread, to become standard in the Middle Ages. It was not until the 2nd century AD that accents and breathings appeared sporadically in papyri. The need for the diacritics arose from the gradual divergence between spelling and pronunciation.
Uncial script
The majuscule, i.e., a system where text is written entirely in capital letters, was used until the 8th century, when the minuscule polytonic supplanted it.
Grave accent rule
By the Byzantine period, the modern rule which turns an acute accent (oxeia) on the last syllable into a grave accent (bareia)—except before a punctuation sign or an enclitic—had been firmly established. Certain authors have argued that the grave originally denoted the absence of accent; the modern rule is, in their view, a purely orthographic convention. Originally, certain proclitic words lost their accent before another word and received the grave, and later this was generalized to all words in the orthography. Others—drawing on, for instance, evidence from ancient Greek music—consider that the grave was "linguistically real" and expressed a word-final modification of the acute pitch.
Stress accent
In the later development of the language, the ancient pitch accent was replaced by an intensity or stress accent, making the three types of accent identical, and the sound became silent.
Simplification
At the beginning of the 20th century (official since the 1960s), the grave was replaced by the acute, and the iota subscript and the breathings on the rho were abolished, except in printed texts. Greek typewriters from that era did not have keys for the grave accent or the iota subscript, and these diacritics were also not taught in primary schools where instruction was in Demotic Greek.