Planar ring molecules with 4"n" π electrons do not obey Hückel's rule, and theory predicts that they are less stable and have triplet ground states with two unpaired electrons. In practice such molecules distort from planar regular polygons. Cyclobutadiene (C4H4) with four π electrons is stable only at temperatures below 35 K and is rectangular rather than square. Cyclooctatetraene (C8H8) with eight π electrons has a nonplanar "tub" structure. However the dianion (cyclooctatetraenide anion), with ten π electrons obeys the 4"n" + 2 rule for "n" = 2 and is planar, while the 1,4-dimethyl derivative of the dication, with six π electrons, is also believed to be planar and aromatic. Cyclononatetraenide anion () is largest all-"cis" monocyclic annulene/annulenyl system that is planar and aromatic. These bond angles (140°) differ significantly from the ideal angles of 120°. Larger rings possess "trans" bonds to avoid the increased angle strain. However, 10 to 14-membered systems all experience considerable transannular strain. Thus, these systems are either nonaromatic or experience modest aromaticity. This changes when we get to , with (4×4) + 2 = 18 π electrons, which is large enough to accommodate 6 interior hydrogens in a planar configuration (3 "cis" double bonds and 6 "trans" double bonds). Thermodynamic stabilization, NMR chemical shifts, and nearly equal bond lengths all point to considerable aromaticity for annulene.