I remember being taught "mothers-in-law" and "trades union" (and also "courts martial") as correct English (although I've never had call to use any of them). Along similar lines does anyone have an opinion on what is the accepted plural of "son of a gun" (and the obvious alternative)? What I was taught suggests "Sons of a gun", but common-sense dictates this can't be right - it would have to be "Sons of guns" because the "guns" would be different...