Compare apples with apples

Means to compare like with like or to make a valid comparison as opposed to comparing apples with oranges, which would be an invalid comparison. The origin is obscure because English Proverbs published in 1670 by John Ray contains the injunction that one should not compare apples with oysters. In French, Spanish, Danish, German etc people talk about not comparing apples with pears. Thus, the fruits that people compare or avoid comparing seem to vary from language to language and culture to culture. These and similar expressions probably date from at least the 16th century.