Original: By then Greek was written left to right, but originally it had been written right to left (with asymmetrical characters flipped), and in-between written either way — or, most likely, in the so-called boustrophedon style, where successive lines alternate direction.
Simplified: 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.