What is "peri-naphthalenes"

In organic chemistry, peri-naphthalenes are particular derivatives of naphthalene with the formula CH-1,8-X. Owing to the rigidity of the naphthalene skeleton, these substituents on the 1- and 8-positions are constrained to be relatively close 2.5 Å, which is within the van der Waals radius for many atoms. In contrast, ortho-substituents pendant from a benzene ring are separated by about 3.3 Å.