The viola tuning is the second important difference between violin and viola and it goes together with the
different
viola size,
which also gives the viola its different (and nicer, in my opinion) timbre, tone quality.
Violin strings, from the highest to the lowest, are:
E, A, D, G.
The viola has the same three lower strings of the violin and instead of the top E it has a lower C string, so the viola tuning is: A, D, G, C.
Sometimes, especially in the baroque and classical era, composers have used different tunings. It was a sort of trick,
called
scordatura
which means mistuning, to improve the sonority of the instrument in special circumstances. See, for example, how
Mozart
used it in his
Sinfonia Concertante
for violin, viola and orchestra. Also Richard Strauss used it, for the viola in Don Quixote as well as
Paganini
in his works for violin.