As I have worked on my 30-year-old house, I found some interior 2x6 wall studs are deeply cut, clearly done at the time the home was built. The cuts are somewhere between 1/3 and 1/2 along the length of the stud.
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I'm guessing that maybe this was done on green studs to prevent the walls from warping as the studs dried, but it's unclear to me if this is #1 common, and #2 whether I should be adding sister studs as I come across these.