A rowlock, sometimes spur (due to the similarity in shape and size), oarlock (USA) or gate, is a brace that attaches an oar to a boat. When a boat is rowed, the rowlock acts as a fulcrum, and, in doing so, the propulsive force that the rower exerts on the water with the oar is transferred to the boat by the thrust force exerted on the rowlock.
On ordinary rowing craft, the rowlocks are attached to the gunwales. In the sport of rowing, the rowlocks are attached to outriggers (often just called "riggers"), which project from the boat and provide greater leverage. In sport rowing, the rowlocks are normally U-shaped and attached to a vertical pin which allows the rowlock to pivot around the pin during the rowing stroke. They additionally have a locking mechanism (properly known as "the gate") across the top of the "U" to prevent the oar from unintentionally popping out of the rowlock.
Originally, rowlocks were two wooden posts or thole pins that the shaft of the oar nestled between. Single thole pins may be used when the oars have holes cut into the loom, which then sits over/around the thole pin.
In sport rowing oarlocks were originally brass or bronze and open (no gate). With the advent of modern materials oarlocks are now injection moulded plastic and precision made to minimize play (slop) between the oar collar and the oarlock. The most recent sport racing oarlocks have a spring loaded feature to keeps the oar collar firmly against the pin at all times.
Oarlocks are technical pieces of equipment in sport rowing, holding the oar shaft and therefore the oar blade at the correct angle in the water to ensure optimum performance.