Simple Tilt tone control

The tilt control imposes a shelving function, which attenuates half of frequency band and augments the other half. In other words, it is special type of a tone control that, unlike the typical tone control that boosts or cuts just the highs or mids or lows, shifts both highs and lows at once.

*Unofficial documentation

source: http://www.tubecad.com/2013/06/blog0266.htm

Credit: Guitarix team