Moog ladder filter
Four transistor pairs stacked like rungs on a ladder — the low-pass filter that defined what an analogue synth is supposed to sound like.
Robert Moog filed for the patent in 1966 and was granted it in 1969. The circuit chains four identical RC stages between transistor pairs, giving a 24 dB-per-octave low-pass filter. Feeding the output back to the input adds resonance; push it far enough and the filter self-oscillates into a sine wave. It also loses bass as resonance rises, a flaw that eventually became part of the character.
The filter lived in the modular systems and the Minimoog, and competitors had to design around the patent. Digitally it's awkward: the feedback loop has no delay in it, which a sampled algorithm can't express directly. Tim Stilson and Julius Smith published a model in 1996, Antti Huovilainen added the transistors' non-linearity in 2004, and modern emulations usually rely on zero-delay feedback.