Most modern bikes have from 1,3,18,21,24 or 27 speeds. To change speeds, slide the shifter generally located on the handle bars. This action changes the chain from one sprocket to another and is called shifting gears or going from one gear to another. Going from a low gear to a high gear is called upshifting. Downshifting is going from a high to a low gear. There are many varieties of bikes with many different varieties of shifters.
Down Tube Shifters
Down tube shifters were the first to come out in the mid-1970s and 1980s. These shifters were placed on the body of the bike below the handlebars. Some old-timers like the look for they think modern shifters add too much clutter to the handlebar area. Down tube shifters change gears with the least amount of parts and shorter cable lengths. This type of shifter operates on friction. They were not that accurate and required a lot of guesswork on behalf of the rider. Down tube shifters no longer are employed by bike manufacturers.
Thumb shifters are mounted on upright handlebars for control. They are similar to down tube shifters in operation with the difference being they are mounted on the handle bars. These type of shifters are prominent on most mountain bikes manufactured during the 1990s. Most bike riders like the convenience of the location of the shifters on the handle bar. Users adapt to using the thumb to shift and most have no problems shifting because the shifters are on the handlebars close to the hands.
Bar Trigger Shifters
Bar-end trigger shifters were made for drop-type handlebars and were developed by Shimano, a leading shifter manufacturer. Their shifter uses the thumb to upshift and downshift at the push of two buttons both attached to the handlebar. At first, downshifting and upshifting were done with just the thumb in the same direction, which caused some confusion. Shimano then came out with “rapid fire plus.” This system employed two different buttons for upshift and downshift mounted on the handlebars and employing the thumb and index finger. This system continues to be in use.