In this figure I have drawn dashed lines to represent cutting lines and walls and the ceiling. The horizontal and vertical lines are ceiling and wall lines respectively. Think of looking at the edge of the ceiling and the edge of the wall. The ceiling runs flat over the client and the wall runs flat behind the client.
The vertical dashed line on the right side shows where the hair would be held to cut it "flat to the wall". In this case it is the wall behind the client, but it works for any flat layering cut to a wall on the side or the back.
The hair is pulled straight out from the head so that it goes right to the wall it is being pulled to. It is not over directed to the middle of the side, front of the side, or back of the side.
Hair cut flat to the side should all reach the wall at the same time as the head moves closer to the wall. If a head shape curves away from a wall, the hair will be longer on the part of the head that is furthest from the wall. For example behind the ear in the mastoid area, hair pulled to the wall behind will be longer than hair pulled from the occipital area.
If as the hair is cut flat to the wall, it is sprayed to keep it in place it should look like a flat brush with the bristles flat at the wall.
The dashed line on top that runs horizontally from left to right is for cutting the hair "flat to the ceiling. " Hair on the top is elevated up to this line and cut on the line which is parallel to the ceiling. The hair is pulled straight up from the top and is not over directed to the middle or sides of the top. The flat top haircut is an example of cutting the hair "flat to the ceiling." In other designs the hair on the top is longer than in a flat top, but it is still cut so that it is "flat to the ceiling."
There is also a curved dashed line which goes around the head and follows the head shape. This can be thought of as a cap that fits the head shape every where. It is elevated off the head shape the same distance at every point of the head. This is supposed to be parallel to the head shape at all places. It is the line for cutting when the length of hair is left at a uniform length from the scalp.
Click on the figure to increase the size.
Where the head shape curves away from the lines for "flat to ceiling" or "flat to wall" cutting, there will be an increase in the hair length that is left. This can be used to smooth the head shape on the sides or top. It can also be used for leaving a quantity of hair for styling.