Why is there in in -in-law instead of by or something similar? Simon Jester, in his answer to this question, quotes the Oxford English Dictionary which says that it originally applied to step-relatives rather than those now considered in-laws, but I still don't know why we use son-in-law instead of son-by-law.
closed as not constructive by FumbleFingers, Jasper Loy, simchona♦, aedia λ, MrHen Nov 4 '11 at 13:33
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In law is a common enough collocation. The OED records 898 examples. Its use in describing relatives is merely a particular case.