H piece

H \H\ ([=a]ch),
   the eighth letter of the English alphabet, is classed among
   the consonants, and is formed with the mouth organs in the
   same position as that of the succeeding vowel. It is used
   with certain consonants to form digraphs representing sounds
   which are not found in the alphabet, as sh, th, [th], as in
   shall, thing, [th]ine (for zh see [sect]274); also, to modify
   the sounds of some other letters, as when placed after c and
   p, with the former of which it represents a compound sound
   like that of tsh, as in charm (written also tch as in catch),
   with the latter, the sound of f, as in phase, phantom. In
   some words, mostly derived or introduced from foreign
   languages, h following c and g indicates that those
   consonants have the hard sound before e, i, and y, as in
   chemistry, chiromancy, chyle, Ghent, Ghibelline, etc.; in
   some others, ch has the sound of sh, as in chicane. See Guide
   to Pronunciation, [sect][sect] 153, 179, 181-3, 237-8.
   [1913 Webster]

   Note: The name (aitch) is from the French ache; its form is
         from the Latin, and this from the Greek H, which was
         used as the sign of the spiritus asper (rough
         breathing) before it came to represent the long vowel,
         Gr. [eta]. The Greek H is from Ph[oe]nician, the
         ultimate origin probably being Egyptian. Etymologically
         H is most closely related to c; as in E. horn, L.
         cornu, Gr. ke`ras; E. hele, v. t., conceal; E. hide, L.
         cutis, Gr. ky`tos; E. hundred, L. centum, Gr.
         "e-kat-on, Skr. [.c]ata.
         [1913 Webster]

   {H piece} (Mining), the part of a plunger pump which contains
      the valve.
      [1913 Webster]