A harmonograph is a mechanical apparatus that employs pendulums to create a geometric image ...
(source: Wiki + BirdandBee)
The most elementary patterns are:
Torus (Open phase - unison 1:1)
Fig. 8 (Open phase - octave 2:1)
Trefoil-knot (Counter current - octave 2:1)
(Source: Wooden book Sacred Number)
