Extended technique

Extended technique is a term used to describe unconventional, unorthodox or "improper" techniques of playing musical instruments.

Examples include:

added electronics or MIDI control
unusual bowing technique: double stops and multiple stops, sul ponticello, sul tasto, Col legno
breath technique or articulation: multiphonics, tonguing or flutter tonguing, continuous breathing or circular breathing, trumpet half-valve playing, humming while blowing, blowing a disengaged mouthpiece or reed, unusual mutes
Sprechstimme (speech-singing)
prepared piano
Unusual harmonics, including multiphonics
glissandi
String microtones (vertical and linear)
exaggerated tremolo
exaggerated brass head-shakes
activating keys or valves without blowing
tapping or rubbing the soundboard of stringed instruments
alternate fingerings

Well known performers and composers who use a notable amount of extended techniques include:

composer Sofia Gubaidulina
vocalist Joan La Barbara
vocalist Shelley Hirsch
vocalist and composer Meredith Monk
composer and multireedist Joseph Celli
pianist and composer David Tudor in his own work and in the prepared piano techniques of Cage and the New York School
cellist and improviser Frances-Marie Uitti, two bows and curved bows (see also Michael Bach )
violinist, violist and improviser Ernesto Rodrigues, curved bow (see also Michael Bach )
rock flautist Ian Anderson
composer Robert Erickson
trombonist Stuart Dempster
bassist Bertram Turetzky

Reading

Stuart Dempster's The Modern Trombone: A Definition of Its Idioms, ASIN 0520032527.
Patricia and Allen Strange's The Contemporary Violin, ISBN 0520224094, and other books in The New Instrumentation series.
Bertram Turetzky's The Contemporary Contrabass ASIN 0520063813.

Woodwind Fingering charts
New Sounds for Flute by Mats Möller
The Orchestra: A User's Manual by Andrew Hugill with The Philharmonia Orchestra. Includes definitions, descriptions and video interviews of extended techniques for most all common orchestral instruments.