Back in the days before SSR in the tower the directors would ring the tower with the landing order. Often the AIR man would ask the No.2 for a space for crossing traffic. Colin Ward (very sadly no longer with us) would warn in something like : "Air France 971, Bealine Echo Mike, Clipper 100, Gold Alpha Papa, Iberia 424". When the AIR man asked where the gap was, Colin would say: "Between Clipper 100 and Iberia 424".. I can still hear his unique laughter now.....