The stem is the bicycle part that connects the handlebars to the fork’s steerer tube. It is a short structural component that transfers the rider’s steering input to the front wheel.
A stem also affects riding position. Its length and angle help determine how far forward and how high the handlebars sit, so changing the stem can alter reach, weight distribution and handling. The handlebar is clamped to one end, while the other attaches to the fork steerer through either a quill or threadless system.
A common mix-up is the head tube. The head tube is part of the bicycle frame and houses the headset bearings; it does not directly hold the handlebars. The down tube and top tube are frame members that connect the head-tube area to the rest of the frame. Older quill stems insert into a threaded steerer, while modern threadless stems clamp around an unthreaded steerer extending above the headset.