The lower third consists of the lower jaw (mandible), upper jaw (maxilla) and the chin. These three features combine together to create a harmonious face. Having even one of these features misaligned can be detrimental to the face.
The lower jaw is the largest, strongest bone in the human face. It's purpose is to hold the lower teeth in place and support the lower face region.
The gonial angle is a crucial part of the lower jaw area. It is the angle from which the ramus and body of the mandible connect. Ideally, the angle should be visibly defined and close to 115-120°. This definition provides the jaw with better angularity and prominence, which is ideal for attraction.
The most important aspect of the lower third is the position of the upper jaw. If the upper jaw protrudes, it will provide an over-bite (malocclusion and if the upper jaw is recessed, it will provide an under-bite (Class III malocclusion). The relationship between the upper jaw and lower jaw provides overall harmony in the face and can make or break the face.
The chin's projection should at least slightly over shoot the glabella. Any shorter and the chin is recessed.
The hyoid bone determines the profile (side view) of the face, it is a bone that resides in the mid-line of the neck.
The hyoid bone differs from each individual and is considered to be genetic determined. Ideally, the hyoid bone should be at-least on the same level with the tip of the chin. If it is seen any lower, a double chin can explicitly be seen, even if the person is not overweight or storing fat on the remainder of their face.
Having a low set hyoid bone contributes to various other health problems, such as sleep apnea, swallowing disorders and lower rest-position of the tongue. 
Blackpill 101 episodeEdit
The importance of the lower third was also picked up and outlined in a Blackpill 101 episode: