This allegedly harks back to a test the Royal Navy did to make sure they were getting proper strength spirits. The spirit (usually rum) would be mixed with gunpowder and a match put to it. If it failed to ignite then it had been watered down. But if it lit, then it was declared 100% proof and good enough for the Royal Navy (and if it exploded it was declared overproof). In modern parlance this is roughly 57% ABV (in American 100% proof means something different and equals 50% ABV). We reckon it was actually just a space-saving exercise, but that’s much less fun. The modern category of Navy Strength gin was introduced by Plymouth Gin in the 1990s. There is no legal definition.