In some cases, the two stereoisomers can interconvert. In cis- and trans-disubstituted cyclopropanones, for example, there is reversible interconversion that favors the more stable trans isomer. This fluxional isomerization occurs via ring opening to an unseen oxyallyl valence bond isomer. As with double bonds, cis and trans isomers are possible, but the restrictions are that W may equal Y and X may equal Z, but W may not equal X and Y may not equal Z. There is an important difference from the double-bond case: The substituted carbons are sterogenic carbons. This means that there are not only two isomers. In the most general case, where W, X, Y, and Z are all different,