One of the sloped boards on either side of a staircase that supports the steps, also called a stringer or string board; a structural element in stair construction.
From 'bridge' (because it spans a space) and 'board' (a wooden plank). The term reflects how staircases were understood as 'bridging' the vertical distance, using the bridge metaphor for spanning.
The term 'bridgeboard' shows how carpenters and builders used the word 'bridge' metaphorically for centuries to describe any structure that spans a gap—this same logic gave us terms like 'drawbridge' and 'footbridge,' all using 'bridge' as a combining element for spanning elements.
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