Moselle has several oscillators. This demo shows off the "Geometric" Oscillator, which isn't band-limited, but rather outputs geometrically perfect triangle, sawtooth, square and the like.
It can do things most oscillators cannot. First, it uses its Duty input (normally used for pulse width on the square wave) to control a smooth morph from triangle to sawtooth wave. Besides modulating this with an LFO or envelope or MIDI controller to get a bit of harmonic motion, you can also simply choose a static value to give your triangle a bit more kick, or your sawtooth a bit less.
The second trick is that it can raise the output curve to a power. The default setting of 1 has no effect, but numbers below 1 cause both triangles and sawtooths to smoothly morph into square waves as the power approaches 0. Using a power higher than 1 creates what we call the 180 degree double-pulse for the triangle, and the 0 degree double-pulse for the sawtooth.
You can of course modulate both the tri-to-saw duty and the power number to keep the waveform ever-changing. Do it with mutually-prime LFOs and it won't even repeat.
Note the sound is recorded directly from Moselle: no hardware or external processing was used.