Beethoven and Mozart did not compose for a single instrument in common the way modern composers sometimes do; their work spans a broad array of instruments, reflecting the musical practices of their respective eras. What their music was written for

• Mozart:
  • Wrote extensively for piano (fortepiano in his time), violin, flute, oboe, clarinet, horn, and various chamber ensembles.
  • Notable instrument-specific outputs include piano concertos and sonatas, violin concertos, wind concertos (for flute, oboe, bassoon, horn, clarinet), and chamber works like string quartets and the famous clarinet quintet.
  • His piano writing was tailored to the fortepianos of the 18th century, which had a more limited range and lighter action than later pianos.
• Beethoven:
  • Also wrote for a wide range of instruments, with a strong emphasis on piano, violin, and chamber ensembles, as well as orchestral works.
  • Early works often use a classical instrumental vocabulary, while his mature period expands the piano’s role and range and pushes the expressive limits of wind, brass, and string instruments.
  • Like Mozart, Beethoven composed for fortepiano on the instruments available in his time, before the modern piano evolved.

Clarifying what you’re after

• If you’re asking for a single instrument associated with both composers, the historical answer is that both wrote for the fortepiano (an ancestor of the modern piano) and for a broad mix of instruments typical of late 18th to early 19th-century practice.
• If you’re looking for a specific instrument each composer favored or for a list of their major works by instrument, I can provide concise catalogs grouped by instrument type.
• If this is for a crossword clue about a 10-letter instrument connected to both composers, the likely answer in modern puzzle sources is “FORTEPIANO,” which historically refers to the keyboard instrument they often wrote for.