Final answer:
For a traditional triangular one length haircut below the shoulders, hair is typically held at 0 degrees elevation to maintain a level perimeter. A slight elevation up to 45 degrees may be used to add shaping around the face.
Step-by-step explanation:
In a triangular one length below the shoulders haircut, the correct elevation at which the hair is held usually depends on the stylist's technique and the client's desired end result. For a classic one-length haircut, the hair is commonly combed down to its natural fall without elevation, meaning 0 degrees, to ensure an even and level perimeter. If however, the aim is to create a slight graduation to frame the face or add a little shape, the stylist might introduce a slight elevation, typically not exceeding 45 degrees. It's important to maintain consistent elevation throughout the haircut to achieve a clean and precise line.