In light-frame construction, a trimmer is a timber or metal beam (joist) used to create an opening around a stairwell, skylight, chimney, and the like. Trimmers are installed perpendicular to floor or ceiling joists and support parallel headers. this is why it is commonly called a "jack" or "jack stud," because it supports or "jacks" other framing elements (just like an automotive "jack" or "car jack", "floor jack" etc. It is lifting and supporting something).
It can also refer to a jack stud that supports a header above a window or door opening.
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