Original: During his adult life, Mendelssohn wrote two piano trios, seven works for string quartet, two string quintets, the octet, a sextet for piano and strings, and numerous sonatas for piano with violin, cello, and clarinet.
Simplified: Piano trios are pieces for piano, violin and cello. A piano quartet would be for piano, violin, viola and cello. String sextets are normally for two violins, two violas and two cellos.