Bend allowance and bend deduction are quantities used to determine the flat length of sheet stock to give the desired dimension of the bent part. Both bend deduction and bend allowance represent the difference between the neutral line or unbent "flat pattern" (the required length of the material prior to bending) and the formed bend. Subtracting them from the combined length of both flanges gives the flat pattern length. The question of which formula to use is determined by the dimensioning method used to define the flanges as shown in the two diagrams below.