box stringn. now rare a stringboard (stringboardn.) which entirely closes in the ends of the steps of a staircase.
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1903 Fireproof Mar. 21/2 (caption) Section Through ‘Box’ String.
1933 County Rev. (Riverhead, N. Y.) 2 Mar. 11/2 The open side of the stair treads away from the wall was covered by a continuous molded member, called the box string, with panels below which concealed the end of the steps.