This trombone music to In Dulci Jubilo is in the key of C. Select the image below for a printable PDF of the melody. The In Dulci Jubilo band play-along page has more versions of this song, so you can play it with other woodwind and brass instruments. The string arrangement of the carol is also in the key of C, so you can play it with violin, viola, cello, and bass. Visit the In Dulci Jubilo lead sheet page which features the carol in seven different keys. On that page you can find an instrumental range or key that is more suitable for you, or print an arrangement that allows you to play along with other instruments.
See the transposition page for explanations and charts to help you determine the appropriate key to play in when performing with instruments that are not in concert key. More Christmas songs for trombone.