In Wilmer Mills' poem, "Diary of a Piano-Tuner's Wife," the constraint of convention that most strikes me is the way the wife in the story sees her husband for who he is, but the people for whom he works (tuning pianos) see a totally different man.
Certainly the piano-tuner must adhere to constraints of convention if he does not want to lose business: he cannot show the same side he exhibits at home, to his customers. When his wife describes the man, she notes that he will go to someone's house, have cake and coffee, and make pleasant conversation as he fixes their piano. Ironically, whereas he may be tuning the piano in another home, his own home is "out of tune." The piano-tuner's wife notices that he cannot feel anything unless it is nastiness:
It’s such a mulish type of sentiment
That governs what a man will take to heart
So only meanness every now and then
Is strong enough to make him stop and feel.
However, the wife's response to circumstance comes from her reaction to her husband. His need to "have straight lines" may arise from wanting a life free from complications and strife: he has already lost his arm in the war in France. However, the wife feels the need to express herself as much as the husband denies any such expression. She will have the lines of the stones she has collected out of order, even if he wants to straighten them. The wife feels a need to express herself freely, where perhaps she has simply been controlled for too long. She realizes that as he keeps "the world in tune," the same can not be said for their home. She says:
Now I know, and so will he: I’m more
Than just another string he fails to tune.
The wife's allusion to her own "Exodus" may refer to her inner-sense of escape and release from the oppressive environment in which she lives, one her daughter won't even return to. The release—if not physical—is at least mental in that the wife will do what she must to help herself find a "life in tune" that her husband cannot or will not provide.