From the link I posted.
The Queen is expected to avoid paying more than £20m in inheritance tax when details of her mother's will and a shuffle of royal residences are announced by Buckingham Palace this week.
The Queen Mother's estate, consisting mainly of valuable works of art, antiques, and race horses is estimated to be worth £50m, and would normally be taxed at the standard rate of 40%. By exercising an obscure "sovereign to sovereign" clause in a 1993 deal with the Conservative government, the Queen, the main recipient, will not pay any tax.
The exemption, which was contested by the late Labour leader, John Smith, also applies to bequests from the consort of a former sovereign to the reigning monarch.