The exchange operator swaps customers in different routes,thereby interchanging two customers simultaneously into the other routes. The k-node interchange by Christofides and
Beasley (1984) is modified by several authors to take time windows into account. Sequentially each customer i is considered and the sets M1 and M2 are identified. M1 is defined as the customer i and its successor j. Then the elements of M2 are found as the two customers closest to i and j but not on the same route as i and j (found by minimizing insertion cost calculated
by the Euclidean distance). A neighbourhood is then defined by removing the elements of M1 and M2 and inserting them in any other possible way. As this neighbourhood is quite large, only the k most promising candidates are checked.