In Beethoven's and Felix Mendelssohn's time in the 19th century, the orchestra was composed of a fairly standard core of instruments which was very rarely modified. As time progressed, and as the Romantic period saw changes in accepted modification with composers such as Berlioz and Mahler, the 20th century saw that instrumentation could practically be hand-picked by the composer. Saxophones were used in some 20th-century orchestra scores such as Vaughan Williams' Symphonies No.6 and 9 and William Walton's "Belshazzar's Feast", and many other works as a member of the orchestral ensemble. Twentieth-century orchestras generally include a string section, woodwinds, brass instruments, percussion, piano, celeste, harp(s), with other instruments called for occasionally, such as electric guitar and electric bass.