Topology study.

In facial topology, it is practical to use edge looping. Edge loops are, loosely defined, a set of connected edges across a surface. Specifically, an edge loop is a loop that follows the middle edge in a junction of four edges. An edge loop is terminated when it meets another kind of junction - for example, a three or five point junction.

Edge loops are a clean and practical way of modelling an organic surface. An edge loop can follow the natural curve of where a muscle would be upon the face, allowing for realistic deformation during animation and sculpting.

When preparing a base mesh for sculpting, it is generally agreed that the use of tris is best kept to a minimum due to the difficulty in subdividing and the occasional oddity that occurs when sculpting onto them. Additionally, five poles and tris should be kept out of major deformation areas due to the animation issues they can cause. However, with a game-ready model, the use of tris is not as big of an issue. Tris are also useful in optimization - one tri can be used in a place where, to keep everything as a quad, many unnecessary edges might have needed to be added.

The picture above (credit to crazyfool of polycount.com) shows some major edge loops good to include within facial topology. They allow easy deformation of the brow line, eyes, mouth (both lips and laughter lines) and jaw during animation and sculpting.