The origins of Parliament go back to the 13th century, and there are many rules and customs that affect how it runs. Some of these are written down and are called 'Standing Orders'. Other rules are set out in resolutions of the House. However, much of how Parliament does its business is not determined by rules but has become established through continued use over the centuries - this is sometimes known as 'custom and practice'.
Standing Orders are written rules under which Parliament conducts its business. They regulate the way Members behave, Bills are processed and debates are organised. Some Standing Orders are temporary and only last until the end of a session or a parliament. There are around 150 standing orders relating to parliamentary business and public Bills, and about 250 relating to private business.
Read the Standing Orders:
Custom and practice
Much of parliamentary procedure is not written into the Standing Orders but exist as the custom and practice of Parliament. Some stem from Speaker's rulings in the House of Commons chamber, other procedures are followed because that's the way things have been done in the past, so a precedent has been set. An example of a well known practice is of Bills being 'read' three times in both Houses, this is not in the Standing Orders.