One of the consequence of Britain having an unwritten constitution is that the House of Commons and the House of Lords often have to rely on unenforceable conventions in their dealings with one another. A convention is an accepted procedure, which has often been followed for a long time, but yet never written down in law.

The Salisbury Convention recognises that the views of the electorate as represented by the Members of the House of Commons may not necessarily coincide with the views of the House of Lords. This convention means that the House of Lords should not reject at second or third reading Government Bills based on election promises. In stead, the Lords should refer a disputed Bill back to the electorate, i.e. back to the Commons.

Although the Convention has roots in the 1900s, its modern form it was introduced by Lord Salisbury, the Conservative leader of the House of Lords. After the end of World War II Labour won the UK general election with a landslide victory. Lord Salisbury believed that, because Labour had such a clear mandate, the House of Lords should not oppose legislation resulting from pledges made to the electorate. Instead of rejecting a Bill that derived from manifesto pledges, the Lords should refer the Bill back to the House of Commons. The House of Lords may still propose reasoned amendments, although 'wrecking amendments' – that is, those designed to destroy the bill – are not permitted.

In recent times the influence of the Convention has decreased. The Liberal Democrats and the Conservatives have indicated that they do not feel bound by the Salisbury Convention, partly as a result of decreasing voter turnout. Manifestos are arguably no longer as decisive as they used to be in influencing voters' decisions, and so the point arises of whether they should still be preserved by this doctrine.

In the Royal Commission on the Reform of the House of Lords' Report of 2000, the House of Lords was recommended to be 'cautious about challenging the clearly-expressed views of the House of Commons on any public policy issue'. The Commission also recommended that where the electorate has chosen a party to form a Government, then the elements of that party's general election manifesto should be respected by the Lords.

Any further House of Lords reform is likely to affect this convention, as well as others. If the Lords' composition is based on democratic choice, then the relationship between the Commons and the Lords is bound to change. Reform notwithstanding the Salisbury Convention lives on, albeit in an ever-evolving state. In the opinion of the Joint Committee on Conventions' Report from session 2005-06, the Salisbury Convention has evolved sufficiently so as to require a new name to reflect the change. The recommended description is the 'Government Bill Convention'.