its aspirated plosive sounds and consonant clusters: Φ (phi) for , Χ (chi) for and Ψ (psi) for . In western Greek variants, Χ was instead used for and Ψ for . The origin of these letters is a matter of some debate. Three of the original Phoenician letters dropped out of use before the alphabet took its classical shape: the letter Ϻ (san), which had been in competition with Σ (sigma) denoting the same phoneme /s/; the letter Ϙ (qoppa), which was redundant with Κ (kappa) for /k/, and Ϝ (digamma), whose sound value /w/ dropped out of the spoken language before or during the classical period. Greek was originally written predominantly from right to left, just like Phoenician, but scribes could freely alternate between directions. For a time, a writing style with alternating right-to-left and left-to-right lines (called boustrophedon, literally "ox-turning", after the manner of an ox ploughing a field) was common, until in the classical period the left-to-right writing direction became the norm. Individual letter shapes were mirrored depending on the writing direction of the current line. Archaic variants There were initially numerous local (epichoric) variants of the Greek alphabet, which differed in the use and non-use of the additional vowel and consonant symbols and several other features. Epichoric alphabets are commonly divided into four major types according to their different treatments of additional consonant letters for the aspirated consonants (/pʰ, kʰ/) and consonant clusters (/ks, ps/) of Greek. These four types are often conventionally labelled as "green", "red", "light blue" and "dark blue" types, based on a colour-coded map in a seminal 19th-century work on the topic, Studien zur Geschichte des griechischen Alphabets by Adolf Kirchhoff (1867). The "green" (or southern) type is the most archaic and closest to the Phoenician. The "red" (or western) type is the one that was later transmitted to the West and became the ancestor of the Latin alphabet, and bears some crucial features characteristic of that later development. The "blue" (or eastern) type is the one from which the later standard Greek alphabet emerged. Athens used a local form of the "light blue" alphabet type until the end of the fifth century BC, which lacked the letters Ξ and Ψ as well as the vowel symbols Η and Ω. In the Old Attic alphabet, stood for and for . was used for all three sounds (correspondinɡ to classical respectively), and