The compass is created by connecting three similar inverse parallelograms.
Similar ratio is 2.
Numbering:
0 for the left first fixed prong,
1 for the next prong, …
and 3 for the last prong,
A1 is angle between prong 1 and prong 0
A2 is angle between prong 2 and prong 0
A3 is angle between prong 3 and prong 0
The compass maintains relation: Ai = i.A1
i = 1 to 3
i.e.: A2 = 2.A1 ; A3 = 3.A1