Introduced in the 14th century, the seat was originally stuffed with English wool, which, due to the importance of the wool trade, was a symbol of the nation's prosperity. When the Woolsack was remade after damage in the Second World War, wool from the various nations of the Commonwealth was used, in order to symbolise the Commonwealth's unity.
The Lord Speaker may speak from the Woolsack when speaking in his or her capacity as Speaker of the House, but must, if he or she seeks to debate, deliver his or her remarks either from the left side of the Woolsack, or from the normal seats of the Lords.
If a Deputy Speaker presides in the absence of the Lord Speaker, then that individual uses the Woolsack. However, when the House meets in the "Committee of the Whole", the Woolsack remains unoccupied, and the presiding officer, the Chairman or Deputy Chairman, occupies a Chair at the front of the table of the House.
In front of the Woolsack is an even larger cushion known as the Judges' Woolsack. During the State Opening of Parliament, the Judges' Woolsack is occupied by the Law Lords. The seat, however, is by no means restricted to judges only; during normal sittings, any Lord may occupy it.
In an episode of Rumpole of the Bailey Rumpole is appalled when it appears his pompous superior is going to "sit on the Woolsack" (become Lord Chancellor).