With substituted benzene rings, an alternative way of identifying the positions of the sub stituents is to use the terms ortho, meta, and para. Ortho compounds are 1,2-disubstituted, meta compounds are 1,3-disubstituted, and para compounds are 1,4-disubstituted. Some examples should make this clear.

The terms ortho, meta, and para are used by chemists because they’re easier to remember than numbers, and the words carry with them chemical meaning. Ortho shows that two groups are next to each other on the ring even though the atoms may not happen to be num bered 1 and 2. They are one example of the way in which chemists don’t always use systematic nomenclature but revert to more convenient ‘trivial’ terms. We consider trivial names in the next section.