The reason why this type of prosthesis is referred to as a removable partial denture is because patients can remove and reinsert them when required without professional help. Conversely, a "fixed" prosthesis can and should be removed only by a dental professional.
Class I RPD's are fabricated for people who are missing some or all of their posterior teeth on both sides (left and right) in a single arch (either mandibular or maxillary), and there are no teeth behind the edentulous area. Thus, Class I RPD's clasp onto teeth that are more towards the front of the mouth, while replacing the missing more-back-of-the-mouth teeth on both sides with false denture teeth, themselves composed of either plastic or porcelain.
Class II RPD's are fabricated for people who are missing some or all of their posterior teeth on one side (left or right) in a single arch, and there are no teeth behind the edentulous area. Thus, Class II RPD's clasp onto teeth that are more towards the front of the mouth, as well as on teeth that are more towards the back of the mouth of the side on which teeth are not missing, while replacing the missing more-back-of-the-mouth teeth on one side with false denture teeth.
Class III RPD's are fabricated for people who are missing some teeth such that the edentulous area has teeth remaining both behind and in front of it. Unlike Class I and Class II RPD's which are both tooth-and-tissue-borne (meaning they both clasp onto teeth and rest on the posterior edentulous area for support), Class III RPD's are strictly tooth-borne, which means they only clasp onto teeth and do not need to rest on the tissue for added support. This makes Class III RPD's exceedingly more secure as per the three rules of removable prostheses that will be mentioned later, namely, support, stability and retention. (See the article on dentures for a more thorough review of these three fundamentals of removable prosthodontics.)
Class I, II and III RPD's that have other edentulous areas in which to replace teeth are further classified with modification states that were defined by Oliver C Applegate.''
Rather than lying entirely on the edentulous ridge like complete dentures, removable partial dentures possess clasps of metal or plastic that "clip" onto the remaining teeth, making the RPD more stable and retentive.
The parts of an RPD can be listed as follows (and are exemplified by the picture above):
In addition there are a couple of specific theories which include the clasp design:
Davis Henderson, Victor L. Steffel. McCRACKEN's Removable partial prosthodontics, 1973. 4th Ed.