The molecule cyclobutane (CB) has a non planar carbon skeleton folded around a line connecting diagonally opposite atoms. The puckering angle (the change from planarity) of similar to 30 degrees is generally attributed to steric repulsion between the four sets of adjacent methylene groups that would be opposed in a planar ring and is relieved by the puckering. According to this criterion, a similar molecule, 1,1,3,3-tetramethylcyclobutene (TMCB), in which adjacent methylene groups do not exist, would be expected to have a planar ring in the equilibrium form. We have investigated the structure of TMCB to test this expectation. Two models were designed for the tests: one having D-2h symmetry (planar ring) and one of C-2v symmetry (nonplanar ring). Each model incorporated the dynamics of large-amplitude bending around a line joining the methylene groups. Our results suggest the D-2h model is to be preferred. Dynamic averages (r(g)/angstrom; <(g)/deg) of the more important distances and angles in the D-2h model with estimated 2 sigma uncertainties, are as follows. = 1.105 (5), C1-C5 = 1.524 (10), C1-C2 = 1.559 (11), C2-C1-C4 = 87.4 (8), C1-C2-C3 = 92.0 (7), C5-C1-C6 = 109.0 (13), and C5-C1-C5 = 115.8 (8). The large-amplitude bending of the ring leads to a thermal average value of the folding angle equal to 177.1 degrees. The results, including the differences between TMCB and CB, are discussed.