Turret clocks are large weight-driven mechanisms built to drive exposed dials, often on more than one face of a tower, and frequently to sound the hours and quarters on bells that may weigh far more than the movement itself. Their frames were traditionally massive cast or wrought iron, built for durability and ease of blacksmith-level repair rather than fine finishing.
The double three-legged gravity escapement, developed by Edmund Beckett Denison (later Lord Grimthorpe) for the Great Clock at the Palace of Westminster (the mechanism commonly, if loosely, called 'Big Ben' after its bell), largely solved a problem specific to turret clocks: driving heavy, wind-exposed exterior hands through a going train without the variable friction of that load reaching, and disturbing, the pendulum's escapement.
In a gravity escapement, the pendulum periodically releases a small arm that has itself been lifted by the going train; the arm then falls under its own weight to give the pendulum its impulse, so variations in the load on the hands affect only how promptly the arm is re-lifted, not the force of the impulse itself, decoupling timekeeping from external load in a way ordinary recoil or deadbeat escapements cannot.
Turret clock movements are conventionally described by their striking arrangement (going train, striking train, and often a separate chiming train for quarters), their escapement type, and their winding interval, since many public clocks were designed for eight-day winding by hand or, from the twentieth century onward, for automatic electric winding of the weights.
From the early twentieth century, many turret clock movements were converted to or built with electric winding or synchronous electric drive, and some historic mechanisms were removed from active timekeeping duty and preserved in place while an electric system took over the public-facing function; a clock's current drive system and its original mechanism can therefore be different objects occupying the same tower.
Firms specialising in turret clock manufacture, such as Gillett & Johnston, John Smith & Sons (Derby), and Vulliamy in an earlier period, are identifiable by nameplates on the frame and by house styles in frame casting and escapement detailing, and archival research into a specific tower clock generally depends on church, civic or company records as much as on the mechanism itself.