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Pop-top Stem
A handlebar stem with a removable face plate, usually held on by two or four bolts. (Also known as a "Pillow-Block" stem.
Pop-top stems permit much easier stem removal/replacement, because they permit the handlebar to be detached from the stem without removal of grips, tape or brake/shift controls. Most newer bikes come with this type of stem.
The "pop top" design also lighter and stiffer.
Traditional aluminum stems are forged or cast, and the horizontal section is solid. The horizontal section is kept relatively thin to keep weight down, but as a result it's less stiff than might be desired.
With a pop-top stem, the manufacturer can use a considerably larger diameter for the horizontal section, and then bore out the middle for lightness. (With a traditional pinch-bolt type stem, there's no access to drill out the horizontal section.)
