An instruction may be mandatory or permissive. There is no formal rule on the political balance of committee membership, and in most cases no fixed number of members. The Chairman of Committees may propose to the House, without reference to the Committee of Selection, Members of the House to fill casual vacancies on select committees. The general rotation rule, subject to variations in respect of certain committees set out below, is that committee members retire after three sessions' service. They are eligible for reappointment after the lapse of one session.
Select committees apply the rotation rule to their sub-committees.
Chairmen of sub-committees are allowed exemption from the rotation rule for up to three sessions after appointment. Chairman 9.14 The chairman of a committee may be appointed by the House on the proposal of the Committee of Selection. Otherwise the Chairman of Committees or, in his absence, a Deputy Chairman takes the chair. In the absence of an appointed chairman, the committee may appoint a substitute. Powers 9.15 A select committee may be appointed to report on a matter referred to it.