Although commonly referred to as the "Thai alphabet", the script is in fact not a true alphabet but an abugida, a writing system in which the full characters represent consonants with diacritical marks for vowels; the absence of a vowel diacritic gives an implied 'a' or 'o'. Consonants are written horizontally from left to right, and vowels following a consonant in speech are written above, below, to the left or to the right of it, or a combination of those.

History

The Thai alphabet is derived from the Old Khmer script (Thai: อักษรขอม, akson khom), which is a southern Brahmic style of writing derived from the south Indian Pallava alphabet (Thai: ปัลลวะ). According to tradition it was created in 1283 by King Ramkhamhaeng the Great (Thai: พ่อขุนรามคำแหงมหาราช). The earliest attestation of the Thai script is the Ram Khamhaeng Inscription dated to 1292, however some scholars question its authenticity. The script was derived from a cursive form of the Old Khmer script of the time. It modified and simplified some of the Old Khmer letters and introduced some new ones to accommodate Thai phonology. It also introduced tone marks. Thai is considered to be the first script in the world that invented tone markers to indicate distinctive tones, which are lacking in the Mon-Khmer (Austroasiatic languages) and Indo-Aryan languages from which its script is derived. Although Chinese and other Sino-Tibetan languages have distinctive tones in their phonological system, no tone marker is found in their orthographies. Thus, tone markers are an innovation in the Thai language that later influenced other related Tai languages and some Tibeto-Burman languages on the Southeast Asian mainland. Another addition was consonant clusters that were written horizontally and contiguously, rather than writing the second consonant below the first one. Finally, the script wrote vowel marks on the main line, however this innovation fell out of use not long after.