If the procedure envisaged in Eccles v Bryant is followed and each contract is concluded only when an exchange through the post is complete, there is a grave risk that X may find that he has committed himself to buy Y 's house, but that W has changed his mind and refuses to buy X's house, with the result that X has two houses on his hands. And Y may find that he has committed himself to sell to X, but that Z has changed his mind, meaning that Y is left without a house at all. The risks are inevitable because of the delay involved in the process of exchange. If the solicitors of all four parties could meet together and exchange the signed parts of the contracts virtually simultaneously, all risk could be eliminated; but it will usually be impracticable for them to meet. A solution has been found in a system of 'exchange' by telephone. If all the contracts have been signed and all the parties are ready to proceed, W 's solicitor may agree to hold the contract signed by W to the order of X's solicitor who in turn will agree to hold X 's signed contract to sell to W to the order of W 's solicitor. X 's solicitor will also agree to hold X's signed contract to buy from Y to the order of Y 's solicitor—and so on down the chain. Some of the contracts may have been physically delivered in advance and held to the order of the delivering solicitor until the time agreed for the 'constructive' (or notional) transfer. But it seems that there need be no physical delivery at all.
The OED lacks the correct definition, as the following do not pertain: