Son-in-Law (22 April 1911 – 15 May 1941) was a British Thoroughbred racehorse and an influential sire, especially for sport horses.

The National Horseracing Museum says Son-in-Law is "probably the best and most distinguished stayer this country has ever known."

[1] Described as "one of the principal influences for stamina in the modern thoroughbred" in Ulbrich's Peerage of Racehorses, Son-in-Law is seen in the pedigree of many of the top 100 show jumpers, particularly those of Holstein heritage.

In 1924 and 1930, Son-in-Law was the leading sire in Great Britain and Ireland.

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