Complex sentence: This instrument was known as the "tenorbass" trombone (German "Tenorbaßposaune")--it was a tenor trombone in B flat with the bore and bell dimensions of a bass trombone, and was used to play both tenor and bass trombone parts.
Split into simpler sentences: This instrument was known as the "tenorbass" trombone (German "Tenorbaßposaune"), since it was a tenor trombone in B flat with the bore and bell dimensions of a bass trombone. It was used to play both tenor and bass trombone parts.