 The SI unit symbols are officially not considered abbreviations and not pluralized, as in 10 m ("10 metres").In "The Language Instinct", linguist Steven Pinker discusses what he calls "headless words", typically bahuvrihi compounds, such as "lowlife" and "flatfoot", in which "life" and "foot" are not heads semantically; that is, a lowlife is not a type of life, and a flatfoot is not a type of foot.When the common form of such a word is singular, it is treated as if it has a regular plural, even if the final constituent of the word is usually pluralized in an irregular fashion.