An electronic approximation of the old studio technique of "double tracking", in which a performer would record the same track twice, note-for-note indentically, and then mix the two tracks. Small differences in tuning between the two performances, owning to normal human and non-electronic instrument variation, would produce a thickening of the part, giving it a more substantial presence in the mix. Double tracking often does not work with synths, because tuning does not vary enough between the two performances (a short period of time usually) to produce the effect. Electronic doubling is done with a delay line, set to a time value of between 20-50 msec. Unlike chorusing and most other delay line effects, doubling is usually done with little or no modulation of the delay time.