Rebar chair (bar support)
A plastic, steel, or concrete support that holds reinforcement at the correct height within the concrete before and during placement.
What does rebar chair (bar support) mean?
Chairs are the difference between reinforcement being where the engineer put it and where gravity puts it, and their spacing must be close enough to prevent sagging under foot traffic. Different types suit different substrates, with wide-base chairs used over vapor retarders and soft subgrade to avoid punching through. On slabs, position within the thickness determines whether the steel does anything at all.
In practice
Skipping chairs and 'hooking' mesh up during the pour is the traditional shortcut, and it reliably leaves reinforcement in the bottom of the slab.