Pair sticks, sometimes called "clapsticks" or "bilma", set the beat for the songs during ceremonies. The rhythm of the didgeridoo and the beat of the clapsticks use patterns that have been handed down for many generations. Traditionally, only men play the didgeridoo and sing during ceremonies, but both men and women may dance. There were female didgeridoo players, but they did not play in ceremonies and it was not encouraged. Linda Barwick, an ethnomusicologist, says that traditionally women have not played the didgeridoo in ceremony, but in informal situations there is no prohibition in the Dreaming Law. On September 3, 2008, the publisher Harper Collins issued a public apology for its book "The Daring Book for Girls", scheduled to be published in October, which openly encouraged girls to play the instrument.