"How do you know where to put the slide?"
     I played bass trombone in high school and college.  Many people, musicians even, asked me the question above.  There are no visibly marked delineations along the slide.  It's a matter of feeling and hearing where the seven positions are.  With practice, the forward part of concentration doesn't pay attention to the positions' locations.  It's like typing, eating cereal, driving, putting Chinese crap in a box on an assembly line: the body learns motions, does all the work eventually without letting the meddling mind intrude.
     When another student musician would ask me the slide-placement question, I'd say, "You just learn it."  I'd also think, if the questioner were a trumpeter, why he or she didn't think about the basic look of the controls of their own instrument--three keys to press down like pistons in various combinations, offering a multi-octave range.
     While driving yesterday, I steered the car around numerous blocked off streets.  Summer road construction and repair will go on until it gets too cold.  Making what seemed to be my twentieth left turn, I noticed my left hand's position on the turn signal, its downward push to activate the left side lights on the car's hull.  Until my attention went there, the action, as usual, had been unconscious.  The term autopilot is often used to describe people "going through the motions," implying a need to get done with a task, in spite of mental and physical exhaustion.  It's a state of mind, in other words, regarded as something unpleasant, the result of circumstances pushing us into strain, and deadness to outer realities.
     Playing trombone, moving the slide where it should go, though mechanical, like driving and typing, has the virtue of eventually allowing musical expression and individual artistry to overtop the necessary foundation of learning early on where it is one puts the slide to sound an F, an E flat, a B.  When the positions are learned and practiced, one doesn't have to think about knowing "where to put the slide."
     Knowing, strangely enough, requires no active thought at all.

                                                                             Vic Neptune