Rivers divide cities and hold them together, often at the same time. The Main runs through Frankfurt with both functions fully operational: the old town on the north bank, Sachsenhausen on the south, and between them a stretch of water that has been the centre of civic life here for close to two thousand years. What that looks like changes by decade, but the basic fact of the river as the city's axis does not.
On a Sunday in July the Mainkai is a continuous human landscape: runners, cyclists, families, teenagers on skateboards, older couples walking slowly alongside the water. Food vendors appear. Music carries from the open-air stages during the festival weeks. The Museumsufer festival in August closes both banks to traffic for three days and replaces the cars with people; it has been happening for forty years and no longer needs to prove itself to anyone.
The bridges
Frankfurt has eleven bridges across the Main within the city limits. The Eiserner Steg, the 19th-century iron footbridge, is the one that appears on postcards. It has padlocks on its railings and a clear view of the skyline, and on summer evenings it is reliably full of people taking photographs of those two things. If you want a better angle with fewer people, walk 300 metres west to the Floßerbrücke. The towers reflect in the water more cleanly from there, the Dom is visible on the right side of the frame, and you can stand at the railing without negotiating for space.
The Alte Brücke, further east, connects Sachsenhausen directly to the old town and is the route most Frankfurters actually use on foot. There has been a bridge at this crossing since the medieval period. Walk across it in the evening and you move from the Museumsufer to the cider pubs in about four minutes, which is one of Frankfurt's more satisfying short journeys.
The window between 30 and 60 minutes after sunset, when the sky is still deep blue and the tower lights are fully on, lasts about 20 minutes on a clear evening. The south bank, from the Floßerbrücke, gives the cleanest view. In October and November, low fog over the Main on clear nights makes the towers glow in a way that can look extraordinary if you happen to be there for it.
The Museumsufer
The south bank between the Alte Brücke and the Friedensbrücke is the Museumsufer: thirteen museums in roughly two kilometres, the result of a deliberate decision made in Frankfurt in the 1980s to concentrate cultural investment along the river. The policy worked, in the sense that the Städel Museum alone justifies a day and the Architekturmuseum and Filmmuseum and several others justify the walk between them. But the Museumsufer's value is also simply the promenade itself. You can move from Renaissance painting to contemporary architecture to film history without taking any form of transport. The river is on one side. The museums are on the other. Between them, benches and trees and the particular pleasure of a city that has decided its public spaces are worth using.
On the water
From spring through autumn, the Primus-Linie runs boat trips along the Main. The hop-on hop-off route goes between the Römerberg landing and Sachsenhausen. It is not a necessary experience. On a warm afternoon, moving slowly along the water with the skyline shifting its angles overhead, it is a rather good one.
The stretch of Mainkai between the Alte Brücke and the Eiserner Steg, on the north bank, fills with people from around 5pm on warm days. Buy something from one of the kiosks, find a section of railing with a view west, and wait for the light to change. This costs nothing and takes about an hour and is one of the better things Frankfurt offers.