Back Perimeter: when all hair in the nape and crown is combed down.

Side Perimeter: when all hair from side and top of head is combed into gravity.

Front Perimeter: when combing down the hair in the front hairline. 

If above the bridge of the nose it's called bangs or fringe.

Concave: in front, back, and side perimeters. Line forms an arc with the curve open at the bottom.

Convex: in front, back, and side perimeters. Line forms an arc with the curve open at the top.

"V": in front and back perimeters. Line forms a V, either strong or slightly angled. Inverted V is the reverse form.

Horizontal: in front, side, and back perimeters. Line is parallel or the floor.

Asymmetrical: in front, back, and side perimeters. Hair is longer on one side than the other.

Forward Diagonal: in side perimeters. Hair on side perimeter is shorter at the the back, getting progressively longer toward the front.

Backward Diagonal: in side perimeters. Hair on the side perimeter is longer at the back, getting progressively shorter towards the front.

Basic Perimeter Areas & Shapes