An airplane navigates through the sky along a route composed of beacons and waypoints. Waypoints are defined by geographic coordinates or their bearing and distance from a beacon, and by a name, which typically takes the form of a five-letter capitalized word—EVUKI, JETSA, SABER. The idea is that they will be pronounceable and distinct to controllers and pilots regardless of their first language. The pilot’s map of the world, and the flight computers’ too, is atomized into these waypoints. They are the smallest nuggets of aerial geography, and in some sense the only such unit that matters once you leave the runway. They are the sky’s audible currency of place.

From a plane, even a wide modern road can look as slow and old-fashioned as an ancient bridleway. The plane slides like an eye over the page, like a finger across a map, over everything the road and the drivers on it must turn to avoid—towns, mountains, lakes—features so low they appear nearly smooth from above. Waypoints, though invisible, remind us that while pilots are not nearly as constrained by the sky as drivers are by roads, neither is our path always as free as it appears.

That is not to say that a waypoint is a place like any other. Though they are often strung together in airways, we’re frequently allowed to move between two distant waypoints without overflying those that lie between; as if a driver could leave the road to tunnel directly through hills and forests before meeting it again, further along. And a waypoint, for all its extraordinary specificity, is not a single place at all. It exists at all altitudes at once. It is possible for many planes to cross the same waypoint at the same time, at different altitudes, yet each plane’s navigation computers show it at the same position. A waypoint is like the address of a skyscraper that does not specify the floor. The speed of a cruising airplane also means that we often do not get anywhere near a waypoint that is on our flight plan, because we must turn well before the waypoint if we are not to overshoot the route on the other side of it. For a sharp turn, in a strong tailwind, we may begin to turn 5 miles before the waypoint, something to imagine, that in a car you would start to turn the wheel so far before the intersection.

There is a rhythm to waypoints, which roughly matches the rhythm of the human geography below. Tourists from North America wandering the cities of Western Europe may have the sense that historically significant places occur every few dozen yards; in the sky over Europe we may cross a waypoint every minute. In contrast, over open sea, or a place such as northern Canada, we may fly forty-five minutes or more, hundreds and hundreds of miles, between waypoints. The pace of passing waypoints also roughly echoes the workload in the cockpit. Most of the waypoints crossed will come in the first and last minutes of a flight, when the plane must make many turns to move between a runway and its route, and then back again at the far end.

Pilots come to know many individual named points on the routes they fly most often. Some, such as those that are well-known entry and exit points for Atlantic Ocean crossings, feel like doors, almost, or gates—when I think of LIMRI or MALOT, off Ireland, I think of the phase of flight in which they occur, the start or the end of an oceanic crossing. The feeling is comparable to the name of a bridge that you only cross when leaving or entering a city, one to which newscasters will casually refer when talking about traffic, and you know they are speaking to those who are leaving town or planning their return.