If carbon atoms 1, 2, 4 and 5 of cyclohexane occupy coplanar positions and when carbon atoms 3 and 6 are on opposite sides of the plane the @C01258@ (of symmetry group D_3d) is called a chair form. The same term is applied to similar conformations of analogous saturated six-membered ring structures containing hetero-atoms and/or bearing substituent groups, but these conformations may be distorted from the exact D_3d symmetry. For cyclohexane and most such analogues, the chair form is the most @S05900@ @C01258@. If the cyclohexane @C01258@ has no centre of symmetry but possesses two planes of symmetry, one of them bisecting the bonds between atoms 1 and 2 and between 4 and 5 and the other plane passing through atoms 3 and 6 (which lie out of the plane and on the same side of the plane containing 1, 2, 4 and 5), that @C01258@ (of symmetry group C_2v ) is called a @B00689@ form and it is generally not a @S05900@ form. Again, this term is also applied to structural analogues. The @C01258@ of D₂ symmetry passed through in the interconversion of two @B00689@ forms of cyclohexane is called the twist form (also known as @S05709@ @B00689@, @S05709@ form and stretched form). In a five-membered ring a @C01258@ in which two adjacent atoms are maximally displaced, in opposite directions, relative to the plane containing the other three carbon atoms has been called a @H02715@ but is better called a twist @C01258@.