Why do we generally leave the second molars off complete dentures?

This increases the stability, retention and support/comfort of the lower denture in three ways: 

1. The second molar is often on the ascending part of the ramus. Should the patient chew a bolus of food on the second molar this can result in a  dislodging  force on the lower denture – pushing the lower denture forwards.

2. The space directly behind the first molar allows the tongue and cheek to rest on the polished surface of the denture increasing the denture stability and also and enhancing lower denture suction/retention.

3. Leaving the second molar off the denture reduces the surface area of teeth for chewing and less biting force is required to chew through the food. The load on the denture bearing area is therefore reduced.