Wikipedia
A T-nut, T nut, or tee nut (also known as a blind nut, which can however also refer to a rivet nut or an insert nut) is a type of nut used to fasten a wood, particle or composite materials workpiece, leaving a flush surface.
It has a long, thin body and a flange at one end, resembling a T in profile. The flanges of T-nuts often have hooks or serrations on the prongs that dig into a wooden work piece as the bolt is tightened from the opposite side of the piece, providing better retention.
In 1969, the first four-prong T-nut with the eight-sided base was patented. Unlike the standard round base T-nuts, the eight-sided T-nuts can be fed efficiently and reliably via machine. The eight-sided T-nut base has become the standard configuration for T-nuts inserted by machine.