The bassoonist may also produce lower notes than the bottom B by extending the length of bell. This can be achieved by inserting a specially made "low A extension" into the bell, but may also be achieved with a small paper or rubber tube or a clarinet/cor anglais bell sitting inside the bassoon bell (although the note may tend sharp). The effect of this is to convert the lower B into a lower note, almost always A natural; this broadly lowers the pitch of the instrument (most noticeably in the lower register) and will often accordingly convert the lowest B to B (and render the neighbouring C very flat). The idea of using low A was begun by Richard Wagner, who wanted to extend the range of the bassoon. Many passages in his later operas require the low A as well as the B-flat immediately above it - this is possible on a normal bassoon using an extension which also flattens low B to B, but all extensions to the bell have significant effects on intonation and sound quality in the bottom register of the instrument, and passages such as this are more often realised with comparative ease by the contrabassoon.