<P> An early use of this notation can be traced back to Richard Kuhn who in 1932 used solid thick lines and dotted lines in a publication . The modern wedges were popularised in the 1959 textbook "Organic Chemistry" by Donald J. Cram and George S. Hammond </P> <P> Skeletal formulae can depict cis and trans isomers of alkenes . Wavy single bonds are the standard way to represent unknown or unspecified stereochemistry or a mixture of isomers (as with tetrahedral stereocenters). A crossed double - bond has been used sometimes; is no longer considered an acceptable style for general use, but may still be required by computer software . </P> <P> Hydrogen bonds are generally denoted by dotted or dashed lines . In other contexts, dashed lines may also represent partially formed or broken bonds in a transition state . </P>

How to tell how many carbon atoms are in a molecule