Illustrated Glossary of Organic Chemistry

One molecule rule: The idea that simple, introductory level spectroscopy problems concern a single, pure substance, and not a mixture of substances. When assembling the pieces to determine the final structure, all of the pieces must be used to make one molecule. If assembly completes a molecule but does not use all pieces, the one molecule rule has been violated.

Given these pieces...

and

and

...assembling the pieces this way does not violate the one molecule rule.

	and
Complete molecule.		Leftover piece.

...whereas assembling the pieces this way violates the one molecule rule.