When smoking a chillum a chillum stone is often inserted in the pipe to keep from directly inhaling debris much like a screen or filter is used in other pipes. They are generally tight fitting, conical with a flat top and usually have a small hole drilled down through the centre and slits down the sides to allow smoke to pass freely. Before they were commercially available people would search for the right size of pebble or stone to fit their pipes, a practice still followed by some.