"Normal" Jaw Opening -

MIO/D:  Maximum Intercisal Opening/Distance or ROM:  Range-of-Motion
Normal MIO:      Men 50-60mm Women 45-55mm
Trismus:    Decreased function of less than 30-35mm MIO.
Basic Diagnostic - The 3-Finger Test:                          
    Insert Tips of index, middle and ring fingers non-dominant hand between front teeth.

Normal MIO	>40 mm	3+ Fingers
Mild Trismus	30-40 mm	2-3 Fingers
Moderate Trismus	15-30 mm	1-2 Fingers
Severe Trismus	<15 mm	<1 Finger

There have been a number of studies on the average and maximum jaw openings of different populations. 

Jaw Function

Muscles

Temporomandibular Joint

Dental Occlusion