The Greek alphabet is thought to be where all important European alphabets came from. Although the alphabet was borrowed from the Phoenicians around the 10th century BC, there were many changes made to make it fit the Greek language. The main change was that some of the Phoenician letters that were for sounds not used in Greek were turned into vowels. The Phoenicians had written their alphabet without any vowels, so this change made writing a lot easier to read. Another change is that some new letters were invented for sounds in Greek but not in Phoenician. At first, Greek was written from right to left, the same as Phoenician, but after the 6th century BC, it was written from left to right.
There were some differences in the early Greek alphabet depending on what part of the Greek world it was used in. The two main kinds were the eastern and western ones. But over time all Greeks started to use the same alphabet, especially after the Ionic alphabet of Miletus was officially adopted in Athens in 403 BC. A little later, the rest of Greece did the same, and by 350 BC, almost all Greeks were using the same twenty-four letter Greek alphabet.
Around this time, Aristophanes of Byzantium invented the three diacritics (accent marks): acute, grave, and circumflex, to mark the tone or pitch of Greek words. Another diacritic is a comma above initial vowels. This signalled whether or not the sound of the letter 'H' was present. This sign is not available in our standard character-set.