Occasionally, numerous teeth may be lost in a patients mouth. This may happen for many reasons; decay, infection, trauma, periodontal breakdown, and sometimes, just bad dentistry.
If tooth loss progresses to the point that there are not an adequate number of teeth remaining to support fixed bridgework, then removable bridgework (partial denture) may become necessary to replace missing teeth. Replacing many teeth with removable bridgework may also be quite a bit less expensive than fixed, permanent bridgework.
As long as even one tooth remains in a dental arch, then the appliance is a partial denture. However, when there are no longer any teeth remaining in an arch, then the removable appliance has become a full denture.
If there are no teeth remaining in either arch, this would then become a set of full dentures, upper (maxillary) and lower (mandibular). Older dentures need to be checked periodically to insure that they still fit and function well.