If you are part of a big chain, we need to get the people in the chain to agree to exchange, without actually exchanging until everyone in the chain has agreed. Otherwise you could have a situation where you’ve agreed to buy the house you want, but the people buying from you haven’t committed to buying your house, which could leave you committed to buying one house, without having sold your house. So the solicitor at the bottom of the chain might call the next person up, and ask them to agree to exchange, and then they are committed to exchange for a period of 2 hours, to allow the second solicitor to call the next solicitor up in the chain, and so on until everyone in the chain has committed to the transaction. If everyone in the chain agrees to exchange during that 2 hour period, then they can all proceed to actually exchange, and everyone is committed to the transaction. However if it isn’t possible to get everyone to agree during that 2 hour period, anyone who has agreed, is automatically released from their promise to exchange, and we will start again the next day. It may sound complicated, but it actually works quite efficiently as the solicitors are well practiced in doing this.