\In`stru*men"tal\, a. [Cf. F. instrumental.]
1. Acting as an instrument; serving as a means; contributing
   to promote; conductive; helpful; serviceable; as, he was
   instrumental in conducting the business.
         The head is not more native to the heart, The hand
         more instrumental to the mouth.       --Shak.
2. (Mus.) Pertaining to, made by, or prepared for, an
   instrument, esp. a musical instrument; as, instrumental
   music, distinguished from vocal music. ``He defended the
   use of instrumental music in public worship.'' --Macaulay.
         Sweet voices mix'd with instrumental sounds.
                                               --Dryden.
3. (Gram.) Applied to a case expressing means or agency; as,
   the instrumental case. This is found in Sanskrit as a
   separate case, but in Greek it was merged into the dative,
   and in Latin into the ablative. In Old English it was a
   separate case, but has disappeared, leaving only a few
   anomalous forms.
{Instrumental errors}, those errors in instrumental
   measurements, etc., which arise, exclusively from want of
   mathematical accuracy in an instrument.