Online linguistics research tools often give data without providing or even suggesting a reason why. Today I was editing a document which included the phrase ‘over the last three decades’. I got wondering whether ‘three decades’ or ‘thirty years’ was the most common. Google Ngrams shows that the latter is, but that its use has been declining since about 1900, and that ‘three decades’ has slowly gaining in use. Searches for ‘two decades’ v ‘twenty years’ right up to ‘nine decades’ v ‘ninety years’ shows the same broad pattern: ‘-ty years’ has been slowly declining and ‘N decades’ has been slowly increasing since about 1900. I can’t think of any reason why.