Cheeks (also known as 'cycloidal cheeks') Cheeks are the shaped pieces designed to shorten the effective length of the pendulum as its amplitude increases, thus ensuring that the period remains constant or isochronal. Cheeks were used because the period of pendulum alters (slightly) depending upon the size of its swing. If the driving force of the clock decreases over time, as is common with spring clocks, the pendulum will swing through a smaller arc and hence have a faster period, making the clock inaccurate. Cheeks were introduced by Christiaan Huygens, who discovered that if the pendulum was not freely hung from a point, but instead trapped between two plates, the changing position of the pendulum pivot point would alter the swing of the pendulum so that it exactly compensated for the change in period, hence keeping the clock accurate. The shape of the plates, or cheeks, is described in mathematics as a cycloid. Cycloidal cheeks are only effective if the clock is accurately set up in a vertical orientation. They were used by many clockmakers, including John Harrison
