The classification refers to Kennedy's classification of partially edentulous arches. Class I describes bilateral edentulous areas located posterior to the remaining natural teeth, which applies when only the first molars remain in the lower arch, leaving bilateral distal extension bases. Modification 1 signifies a single additional edentulous space anterior to the remaining teeth but does not change the primary class. Since only the first molars are left, the areas distal to them are edentulous bilaterally, fitting Class I modification 1. This classification guides appropriate prosthesis design for bilateral distal extensions.

Reference: Contemporary Fixed Prosthodontics, Rosenstiel NF, 5th Edition.