What is "pedal surface"

Pedal \Pe"dal\, a. [L. pedalis, fr. pes, pedis, foot. See Foot, and cf. Pew.]

1. Of or pertaining to the foot, or to feet, literally or figuratively; specifically (Zo["o]l.), pertaining to the foot of a mollusk; as, the pedal ganglion.

2. Of or pertaining to a pedal; having pedals.

   Pedal curve or Pedal surface (Geom.), the curve or surface which is the locus of the feet of perpendiculars let fall from a fixed point upon the straight lines tangent to a given curve, or upon the planes tangent to a given surface.

   Pedal note (Mus.), the note which is held or sustained through an organ point. See Organ point, under Organ.

   Pedal organ (Mus.), an organ which has pedals or a range of keys moved by the feet; that portion of a full organ which is played with the feet.