Biangle

If n planes be drawn through any axis of the right-vector, each of which makes angles π/n with the planes on either side of it, the whole of space is divided into n congruent figures which may be called biangles, the space between any two adjacent planes being easily seen to be continuous with the vertically opposite space between them.

Source: wiktionary

Then the right biangle CABD and the oblique biangle CPND are equivalent since the triangles API and BNI are congruent.

Source: wiktionary

It is therefore natural to consider whether precisely this concept of a straight line, with which Kant¹¹ too was familiar, is the reason why Kant in the one passage says that the concept of a straight biangle is free of contradiction.

Source: wiktionary