[overconfident(?) comment by someone who has taken exactly one (1) linguistics class]<p>That seems prescriptivist.<p>If people take &quot;Mother-in-law&quot; to be a single word, then it is a single word. And if they take &quot;Mother-in-laws&quot; to be the plural of this word, then it is the plural of that word.<p>Language is determined by usage. People do not say &quot;s-in-law&quot; , they say &quot;in-laws&quot;.<p>I don&#x27;t mean that you can&#x27;t have a preference that usage be a particular way. I might even encourage it. If you prefer people using &quot;mothers-in-law&quot; over &quot;mother-in-laws&quot; for the reasons you gave, that&#x27;s fair. Go ahead and try to convince people to say it that way. Maybe they will agree with you, that doing it only that way has advantages. I can certainly see doing it that way having advantages in some cases.<p>But for it to be &quot;wrong&quot;, there has to be some standard for it to be &quot;wrong&quot; by.<p>When speaking about english as it is spoken, the standard is pretty much the ways in which people speak it.<p>[&#x2F;overconfident(?) comment by someone who has taken exactly one (1) linguistics class]