Remark 3

Given two lines (resp. half-lines) $D, D'$ of an affine plane $L$ attached to $E$, the angle of $D$ and $D'$ is defined to be, and is denoted by $(\widehat{D, D'})$, the angle made by their directions in $E$ (resp. by the half-lines with origin 0 of $E$ corresponding to $D$ and $D'$) (Chapter II, 2nd ed., App. II, No. 1 and No. 3).