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Webster 1913 Edition

Noun.

(Arch.)

(a)

A long piece of timber, forming a margin or edge of any piece of construction; esp.:

(b)

One of the longitudinal pieces, supporting the treads and rises of a flight or run of stairs.

stringpiece ‎(plural stringpieces)

(architecture) A long piece of timber, forming a margin or edge of any piece of construction; especially one of the longitudinal pieces supporting a flight or run of stairs.