The Chock
A chock on a ship is a heavy fitting — U-shaped, horn-shaped, or fully enclosed — bolted to the deck near the rail. Mooring lines pass through it on their way from the ship to the pier. The chock guides the line at the correct angle, prevents it from sawing against the rail, and defines the point at which the ship's geometry meets the dock's. A vessel may have a dozen chocks along its length, each positioned for a specific mooring configuration: bow lines, spring lines, stern lines. The chock does not hold the line. The bitts or cleats do that. The chock tells the line where to cross the boundary between ship and shore.
A wheel chock is a wedge placed against a tire to prevent a vehicle from rolling. Wood, rubber, molded plastic, or welded steel — the shape is always a triangle, the low edge against the tire, the high edge against the ground. Aircraft on a ramp must be chocked immediately after the engines shut down; a heavy airplane on even a slight grade will begin moving, and once it moves it cannot be stopped by a person standing in front of it. Trucks are chocked during loading. Rail cars are chocked during switching. The chock provides no braking force. It is not a brake. It is the object that is present when the brakes are absent, unavailable, or insufficient — the last physical barrier between a stationary vehicle and the slope that wants to move it.
Chock-a-block is a nautical term for the condition when two blocks in a tackle system have been hauled so close together that no more line can be taken in. The hauling part goes slack because there is nowhere left to pull. The system is at maximum tension, maximum mechanical advantage, and zero remaining travel. Colloquially the word means full, packed, jammed tight — a bus chock-a-block with passengers, a schedule chock-a-block with meetings. The original meaning is more precise: not merely full but at the limit of the mechanism. The blocks cannot come closer. The line cannot be shortened. Whatever the tackle was doing, it has done all of it.
Three chocks: a deck fitting, a wheel wedge, a state of maximum tension. Each one names a limit — the boundary where the line crosses the rail, the stop that prevents rolling, the point where the tackle runs out of travel. The chock is always about where something ends: where the rope leaves the ship, where the wheel meets its constraint, where the system has no more room. It is the fitting that marks the edge of the available.