The Associated Board of the Royal Schools of Music issues syllabuses for 'viola and piano' at three levels of difficulty.
In these works the viola and piano parts are both at about the same level of difficulty.
PRIMARY viola and piano ensembles
(UK Grades 4 to 5, ASTA Grade 3)
C. P. E. Bach Sonata in G minor, Wq.88
International-Kalmus/M.D.S.
Howard Ferguson 4 Short Pieces : complete
Boosey and Hawkes
Michael Miller 3 Miniatures : complete
Peters
Pepusch Sonata in D minor
Schott/M.D.S.
R. Valentine Sonata no.9 in A minor
Schott/M.D.S.
Vaughan Williams 6 Studies in English Folk Song : nos.1, 2,
3 and 4
Stainer and Bell
INTERMEDIATE viola and piano ensembles
(UK Grades 6 to 7, ASTA Grades 3 to 4)
J. S. Bach Viola da Gamba Sonata no.1 in G, BWV 1027
from Bach 3 Gamba Sonatas
Peters
Flackton Sonata in C minor, Op.2 no.8
Schott/M.D.S.
Hindemith Trauermusik
Schott/M.D.S.
Herbert Murrill 4 French Nursery Songs : complete
Chester/Music Sales
Hans Poser Sonatine, Op.54 no.3
Sikorski/William Elkin
Telemann Sonata in A minor
Peters
ADVANCED viola and piano ensembles
(UK Grade 8, ASTA Grades 4 to 5)
J. S. Bach Viola da Gamba Sonata no.3 in G minor, BWV 1029
from Bach 3 Gamba Sonatas
Peters
Bloch Suite Hebraique
Schirmer/Music Sales
Bruch Romance for Viola, Op.85
Schott/M.D.S.
Glinka Sonata in D minor
Musica Rara-Kalmus/M.D.S.
Handel Sonata in G minor
Schott/M.D.S.
Telemann Sonata in D
Peters