What was the rationale behind the keyboard codes for the symbols?

    It may be easier to remember the keyboard codes if you know why they were chosen, so here are the explanations...

1) First, the letters a-g and A-G are reserved for jumping to staff lines (eg. "c" jump the caret down to the nearest "c" staff line, while "C" jumps the caret up to the closest "c"), so they couldn't be used for symbol codes

2) Notes:
      w = whole note,     h = half note,    q = quarter note,    s = sixteenth
       t = eighth note ( "e" was reserved, see note 1 above)
       y = thirty second note ( "t" was taken by eighth note )
       x = sixtyfourth note ("s" was taken by sixteenth note, and x is unique to "sixty fourth")

3) Codes which look very much like their symbol:
     . (dot)                  : (multi-dot)             > (accent)              # (sharp)
     ( (left paren)         ) (right paren)          ~ (trill)                   | (bar)
     % (repeat bar)    * (lift pedal symbol)   + (tap)                  o (un-tap),
     / (up slide)           \ (down slide)           @ (number in a circle)
     p (p pluck)           i (i pluck)                 m (m pluck)     note: "a" was taken (see note 1)

4) Codes which look somewhat similar to their symbol:
     $ (grace note)             ; (stacatto)           ? (fermata)            _ (whole rest)
     - (half rest)                  ! (quarter rest)    & (flat)              &+& (double flat)
     ^ (mordent)                = (tremlo)            z (glissando)
     n (down curved tie)     u (up curved tie)  < (up stroke)
      " (double bar)             I (end bar)           [ (left bar)            ] (right bar)
      P (Ped symbol)          O (coda)              Z (diagonal "strum" slash)            W (vibrato),

5) Codes which were left over and assigned for other reasons (or randomly):
     L (legato)                        ' (eighth rest)               ` (sixteenth rest)
     j (thirty second rest)        J (sixty fourth rest)       N (natural)
     R (pre-bend)                  { (up bend)                   } (down bend),
    k (1/2 bend - looks a bit like a fraction)     k+k (1/4 bend)      K (Full bend)
    U (treble clef)              U+U (bass clef)                 l (slide)
    M (eighth pair)            M+M (shuffle rhythm)
     v (hammer on - down arrow, like hammer motion)   V (pull off - same key as hammer on)
     r ( ring finger, "a" of p,i,m,a but "a" was taken already - see note 1 )
     , (down stroke - same key as up stroke)
 
6) Note styles:
      Alt + n  =  normal notes (notes are "o" shaped, but Alt + o activates the Options menu! )
      Alt + /   =  strum notes ( shaped like a / )
      Alt + x  =  muted notes (shaped like an x )
      Alt + r   =  harmonic notes ( Alt + h activates the Help menu! )
      Alt + m  =  mini notes
      Alt + q  =  square notes
      Alt + g  =  triangular notes
      Alt + c  =  x'd circle notes