However, the upper ring-finger key can be used, typically for B2 and B3, in place of the top thumb key on the front of the boot joint; this key comes from the oboe, and some bassoons do not have it because the thumb fingering is practically universal. The smallest finger operates three keys.
This passage describes a specific mechanism in musical instruments, particularly bassoons and oboes. It explains how the upper ring-finger key can be used as an alternative for the top thumb key on the front of the boot joint. This key is commonly found on oboes but not always on bassoons. The smallest finger controls three keys.