Chair
Chair turns up in 2 fields: concrete, railroad. Here is what it means to the people in each one.
Chair in concrete
A wire or plastic stand that holds rebar or mesh at the right height inside the form.
Bar supports keep reinforcement off the subgrade or form face so it ends up with the cover the drawings call for. Chairs come in set heights such as 1.5, 2 and 3 in, with variants like slab bolsters, high chairs and plastic-tipped chairs for exposed surfaces. Too few and the mat sags when crews walk it; the wrong height and the cover is wrong for the life of the structure.
"Put a chair every four feet under the bottom mat, the inspector already flagged bar laying on the vapor barrier."
Chair in railroad
A heavy cast-iron bracket that holds bullhead rail in place on the crossties.
A metal casting bolted or spiked to the sleeper, featuring a jaw that grips the web and lower loop of a bullhead rail. A wooden or steel key is driven between the rail and the chair to lock the track in gauge. North American railroads favor flat-bottomed rail secured by tie plates, making chairs primarily a feature of British trackwork or older historic installations.
"The track gang is driving new keys into the chairs on the heritage line."