The first chronometric instruments were based on the changing direction and length of shadows cast by objects exposed to sunlight. The shadow of an indicator (called style or gnomon) falling on a set of hour lines showed the time. Alternatively, the Sun's height above the horizon could be measured directly. Thanks to the mastery of projective geometry, by the 16th century instrument makers were producing many different kinds of sundials - instruments that could be suspended from a hook, placed on shelves or mounted on walls, as well as pocket instruments. Some of these are still in use today.