A wall-mountable programmable electronic thermostat for controlling an HVAC system is described. The thermostat includes a circular wall-mountable backplate with a central opening to allow for the passage of HVAC wires for electrical connection to the thermostat. The head unit body is also circular and is removeably mountable to the back plate. A plurality of wedge-shaped wiring terminals are mounted on the backplate for making a tool-free connection to HVAC wires. Each wiring terminal has button that a user can depress while a wire is inserted in a wire hole. The terminals are arranged along one or more circular arcs about the central opening of the backplate such that the wire holes face the central opening and the buttons are located close to the outer periphery of the backplate.