The Joint Committee (Article 53 a GG) is an emergency parliament, which exercises the power of Bundestag and Bundesrat, if the Bundestag can not meet (in particular in the case of defense).
Two thirds of its members are members of the Bundestag and one third are members of the Bundesrat. The members of the Bundestag are determined according to the relative strength of the parliamentary groups and are not allowed to belong to the Federal Government. The members of the Bundesrat, one per Land, are not bound at instructions. Because there are 16 Laender now the Joint Committee has 48 members.
Article 53 a
(1) The Joint Committee shall consist of Members of the Bundestag and Members of the Bundesrat; the Bundestag shall provide two thirds and the Bundesrat one third of the committee members. The Bundestag shall designate Members in proportion to the relative strength of the various parliamentary groups; they may not be members of the Federal Government. Each Land shall be represented by a Bundesrat Member of its choice; these Members shall not be bound by instructions. The establishment of the Joint Committee and its proceedings shall be regulated by rules of procedure to be adopted by the Bundestag and requiring the consent of the Bundesrat.
(2) The Federal Government shall inform the Joint Committee about its plans for a state of defense. The rights of the Bundestag and its committees under paragraph (1) of Article 43 shall not be affected by the provisions of this paragraph.