If a Bill is amended in the Commons, it must return to the Lords for peers to endorse or reject changes made there. The Government tables motions agreeing or disagreeing with Commons Amendments. It may also move amendments in lieu of changes rejected. Given the balance of power in Parliament, the Lords are normally considering the insistence of the Commons on certain provision struck out by the Lords.
Unless the Lords acquiesce with the provision agreed by MPs, the Bill will be passed back to the Commons for their consideration. The Bill is communicated with 'Messages', which outline why peers do not agree with the Lower House.
If the Commons still cannot agree to the provision, they re-amend it and send it back (they communicate with the Lords via 'Reasons'). The Lords consider the changes and may again reject them or make further changes in lieu. The Bill goes back to the Commons.
In theory, this so-called 'ping-pong' can continue indefinitely until consensus is reached but this is limited by the availability of Parliamentary time, the length of the Session and the likelihood that one or other side will put forward an acceptable compromise. In most cases, if a Bill does not receive Royal Assent before prorogation, it is lost.
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