The retromolar space or retromolar gap is a space at the rear of a mandible, between the back of the last molar

Molar (tooth)

Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....

 and the anterior edge of the ascending ramus where it crosses the alveolar margin

Alveolar ridge

An alveolar ridge is one of the two jaw ridges either on the roof of the mouth between the upper teeth and the hard palate or on the bottom of the mouth behind the lower teeth. The alveolar ridges contain the sockets of the teeth....

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Retromolar gap is found in some Neandertals because they had midfacial prognathism

Prognathism

Prognathism is a term used to describe the positional relationship of the mandible and/or maxilla to the skeletal base where either of the jaws protrudes beyond a predetermined imaginary line in the coronal plane of the skull. In general dentistry, oral and maxillofacial surgery and orthodontics...

. This caused the lower toothrow to move forward, which led to the retromolar gap.

The pear shaped pad is a triangular shaped area of keratinized tissue that forms from the scarring after removal of the most distal molar. The retro molar pad is a non keratinized area of tissue and is a posterior continuation/extension of the pear shaped pad. It is a small inclination going up and posteriorly and is bordered by muscles in the back of the jaw. The denture base should only extend 1/2 to 2/3 up the retro molar pad.