Violin strings, from the highest to the lowest, are: E, A, D, G.
Three of the viola strings are the same as the three lower strings of the violin and instead of the top E it has a lower C string, which is the most beautifully sounding of the four strings. 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
which means mistuning, to improve the sonority of the instrument in special circumstances. See, for example, how
used it in his
for violin, viola and orchestra. Also Richard Strauss used it, for the solo viola part in Don Quixote as well as
in his works for violin.