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Thumb Shifter
From WikiPedalia
The original style of shifter used on mountain bikes, up until the early '90s.
When the Shimano Rapidfire ® and Sun Tour X-Press ® shifters hit the market, confusion resulted, because these, too were operated by the thumbs. This confusion persists, so it is a good idea to avoid the term "thumb shifter." The newer term for these is "top-mount" shifters, as opposed to "below-the-bar" shifters, such as Rapidfire.
