Note: AP means Ancient Pronunciation. MP means Modern Pronunciation. These represent the phonetic values of the Greek letters in Classical and modern times.

While there are many differences between the many variants of the early Greek alphabet, enough similarities exist to suggest the Greeks adopted the Phoenician alphabet once and splintered rapidly into local variants rather than adopting multiple times.

Early Greek was written right-to-left, just like Phoenician. However, eventually its direction changed to boustrophedon (which means "ox-turning"), where the direction of writing changes every line. For instance, you start on the right of the tablet and writes leftward, and when you reach the leftmost end, you reverse your direction and starting writing toward the right. Even more confusing is that the orientation of the letter themselves is dependent on the direction of writing as well. In the above chart, the letters are drawn as if they were being written from left-to-right. If I were to write right-to-left, I would horizontally flip the letters (like in a mirror).

Boustrophedon was an intermediate stage, and by the 5th century BCE, left-to-right became the de-facto direction of writing.

The Greek alphabet was also the basis for Glagolitic, Cyrillic, and Coptic, Gothic scripts among others.

Strangely, the Greeks tried writing once before. Between 1500 and 1200 BCE, the Mycenaeans, an early tribe of Greeks, has adapted the Minoan syllabary as Linear B to write an early form of Greek. However, the syllabary was not well suited to write Greek, and leaves many modern scholars scratching their heads trying to figure out the exact pronunciation of Mycenaean words. The alphabet, on the other hand, allowed more precise record of the sounds in the language.