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