A major

A major (or the key of A) is a major scale with a base note of A. Its key signature has three sharps.
Its relative minor is F-sharp minor. The key of A-major is the only key where a Neapolitan sixth chord on formula_1 needs both a flat and a natural accidental.
There are not as many symphonies in A major as in D major or G major, but more than other sharp keys. Beethoven's Symphony No. 7, Bruckner's Symphony No. 6 and Mendelssohn's Symphony No. 4 are almost all the symphonies in this key in the Romantic era. Mozart's Clarinet Concerto and Clarinet Quintet are both in A major. Mozart used clarinets in A major often.
In chamber music, A major is used a lot. Johannes Brahms, César Franck, and Gabriel Fauré wrote violin sonatas in A major. Peter Cropper said that A major "is the fullest sounding key for the violin.", when he was talking about Beethoven's Kreutzer Sonata.
According to Christian Friedrich Daniel Schubart, A major is a key that is good for "declarations of innocent love, ... hope of seeing one's beloved again when parting; youthful cheerfulness and trust in God."
When music for orchestra is in A major, the timpani are normally set to A and E a fifth apart. In most other keys, they are set a fourth apart.