Function f(x), LLC is a guitar effects pedal manufacturer headquartered in Elk Grove Village, Illinois, just outside Chicago, with additional research and development in New Jersey. The company was co-founded by Dave Friesema (known in pedal communities as "pickdropper"), and all of its pedals are designed and assembled in the United States using modern rather than vintage or obsolete components.

The brand is particularly known for its line of mini pedals built in 1590A-sized enclosures, spanning fuzz, overdrive, envelope filter, octave, tremolo, and phaser effects. Notable models include the Clusterfuzz (a medium-gain overdrive to square-wave fuzz), the Cannon dual fuzz (combining Tycobrahe Octavia and AstroTone Fuzz-style circuits), the Gate Crasher octave fuzz, the Accufunkture envelope filter (derived from the Mutron Micro V circuit), the Parallax tremolo/phaser mashup, and the Third Rail dual overdrive.

Function f(x) has developed a following among boutique pedal enthusiasts through community threads on The Gear Page and coverage on sites like Guitar Pedal X, which has profiled Friesema's work on multiple occasions.

Function f(x) has 7 products cataloged on Equipboard, including Fuzz Effects Pedals, Overdrive Effects Pedals, and Tremolo Effects Pedals. Their gear is featured by 2 artists, with the strongest followings in Rock, Blues, and Funk / Soul. Notable users include Gary Clark Jr and Andy Martin.