The earliest reasonably accurate mechanical clocks are the 13th century tower clocks probably developed for (and perhaps by) monks in Northern Italy. Using gears and gradually falling weights, these were adjusted to conform with canonical hours—which varied with the length of the day. As these were used primarily to ring bells for prayer, the clock dial likely only came later. When dials were eventually incorporated into clocks, they were analogous to the dials on sundials, and, like a sundial, the clocks themselves had only one hand.