bar

Sense pinned: bar as its accepted definition carves it — the notational unit of musical time, delimited by bar lines on a staff and counted by the time signature. Directional test (Law 7, which-came-first, from independent history): early neumatic notation (9th-10th c.) carried no measures at all; the discrete measure emerged out of musical notation's own development — 13th-c. mensural notation's proportional units, then the bar line as a visible boundary (attested c. 16th c.), which fixed the recurring unit the time signature governs. Musical notation existed first and fed the bar into existence; in the definition's own pinned sense (delimited by bar lines, beat-counted by time signature) the bar is a product of notational practice, not a pre-notation temporal unit that notation merely recorded. Y-existed-first-and-fed-X holds.

Pinned sense: bar as measure unit delimited by bar lines on staff. The definition states a bar contains exactly the number of beats specified by time signature. Removing beats stops bar operating as a measure unit; beats are literal temporal parts contained within the bar's span. Law 10 whole-and-part.

sheet music as a document type contains bars as structural units — a bar (measure) is a constituent element of a sheet music document's layout. Law 10 CONTAINS: whole contains part.