Complex sentence: It derives from the Greek, the first script to consistently assign letters to both consonants and vowels, which in turn was derived from the Phoenician alphabet (it was an abjad-a system where each symbol usually stands for a consonant).
Split into simpler sentences: It derives from the Greek, the first true alphabet in that it consistently assigns letters to both consonants and vowels. The Greek alphabet in turn was derived from the Phoenician alphabet (it was an abjad-a system where each symbol usually stands for a consonant).