The Royal Thai General System of Transcription (RTGS) is the official system used to write Thai words into the Latin alphabet, or romanization. Thai has certain vowel and consonant sounds and other phonemic features that do not exist in English. This means that saying a syllable with a different tone or vowel length can make a completely different word. For these reasons, romanization in Thai presented many challenges. The RTGS only uses the 26 letters in the ISO standard Latin alphabet to spell Thai words. It is used on all documents that require words to be written in the Latin alphabet, including road signs, maps, applications, etc. However, it has many problems. The system does not show tones or vowel length, both of which are phonemic features of Thai. It also does not tell some of the consonant sounds apart.