Which of the following is NOT a type of straight line used in haircutting?

The choice indicating "straight" as a type of line is not accurate within the context of haircutting terminology. In haircutting, the lines commonly recognized are vertical, diagonal, and horizontal. Each of these lines serves specific purposes in shaping and structuring the hairstyle.

Vertical lines are used to create length and can give the appearance of added height. Diagonal lines are often utilized to create movement and softness or to blend various lengths within a cut. Horizontal lines, on the other hand, are typically employed to add width and to create a balanced look. The term "straight," while descriptive of the nature of lines, does not represent a distinct category or application in haircutting principles. Hence, it does not fit in the context of haircutting classifications as the others do.