Sidney Lau is a system of romanisation for Cantonese, developed in the 1970s by Sidney Lau for teaching Cantonese to Hong Kong Government expatriates. It is based on the Hong Kong Government's Standard Romanisation which was the result of the work of James D Ball and Ernst J Eitel about a century earlier.
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Innovation
Lau's singular creative step was to indicate tonality with superscript numbers so as to do away with diacritics (with just one exception) entirely. His system was a plain attempt at simplification which proved popular with western learners of Cantonese as a second language and was initially the system of romanisation adopted by the University of Hong Kong. However, the university now employs the similar Jyutping system for its Cantonese courses.
Finals
In his system, Lau treats [ɵ] and [ʊ] as allophones of one phoneme represented with "u", while they are often respectively regarded as allophones of /œ/ and /u/ in other systems.
Tones
Seven tones are indicated using superscripted numbers.