The first 'classic' ruleset endorsed by both CAMREC and IMCS at the great MC negotiations. The rulest came into being on June 4th 1931 and is the basis for most modern rulesets ([Chalk Farm 83]?, 84, Finsbury Option and Holland Park 2000 included).
It was innovative in recognising moves which had previously been only defined in terms of their exponents: the Chudley-Smith progression was recognised as the first "cross-hatch", and the word 'pickering' was used for the first time in a ruleset.
Crescent '31 was also the first ruleset to be used uniformly across the grand-slam tournaments, and was finally recognised as the World-Standard in 1935, when the "All-England Mornington Crescent Club" was amalgamated from the central London clubs. [Edward Cholmondeley-Davis]? won the World Championship that year, the first champion under Crescent '31.
Despite many new rulesets that came in the late-30s-40s ([Crescent '38]?, [Praed Street '41]?, [Mark Lane '46]?) Crescent '31 continued as the ruleset of choice for many tournaments, including the World Championship (although Praed Street '41 was used for the 42-43 tournament) until it was superseded by [Marble Arch '54]?. Aficionados still like to resurrect Crescent '31, although in recent years it has fallen into increasing obscurity.