p-xylene is an aromatic hydrocarbon. It is one of the three isomers of dimethylbenzene known collectively as xylenes. The p- stands for para-, implying that the two methyl groups in p-Xylene occupy the diametrically opposite substituent positions 1 and 4, as illustrated here in the schematic representation. It is in the positions of the two methyl groups, their arene substitution pattern, that it differs from the other isomers, o-xylene and m-xylene.