4-(4-Chloro-phen-yl)-8-methyl-2-oxo-1,2,3,4,4a,5,6,7-octa-hydro-quinoline-3-carbonitrile.

The six-membered N-heterocyclic ring of title compound, C(17)H(17)ClN(2)O, is fused with a methyl-substituted cyclo-hexene ring. The nitro-gen-bearing ring has an envelope conformation with the benzene ring-bearing C atom lying 0.432 (6) Å out of the plane defined by the other five atoms (r.m.s. deviation 0.011 Å); its benzene substituent is aligned at 84.7 (1)° to the latter plane. The cyclo-hexene ring adopts a half-chair conformation. In the crystal, two mol-ecules are linked about a center of inversion by pairs of N-H⋯O hydrogen bonds, generating dimers. An ethyl-ene portion is disordered over two orientations in a 1:1 ratio. The crystal studied was a non-merohedral twin with a 15.3 (1)% minor component.