Sidney Lau / 劉錫祥式粵語拼音方案
Sidney Lau (traditional Chinese: 劉錫祥, died 1987) wrote a series of textbooks in the 1970s, for teaching western people to speak Cantonese. The textbooks were initially used for teaching western expatriates working in the Hong Kong Police Force and other government bodies. Later the texts were used as a basis for a radio teaching programme for foreigners.
They introduced Lau's own romanisation system which differs from the Yale system by using superscripted numbers and symbols to indicate the word tones.
Despite their age, the books are still very popular being one of the few comprehensive books teaching spoken Cantonese (as opposed to written Chinese and spoken Mandarin, which are significantly different).
[edit] Other links
Explanation of Sidney Lau's Cantonese Romanization System
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http://en.wikipedia.org/wiki/Sidney_Lau