Look up at almost any canal house in Amsterdam and something is slightly off. A facade leans forward. A roofline sits a few degrees out of true. None of it is an accident, and none of it is decay.

Why do so many canal houses lean forward?

Builders tilted the top of the facade outward on purpose, usually by a few degrees. Staircases inside these houses are steep and narrow, often too tight to carry furniture up. The forward lean lets a beam and pulley haul sofas and pianos past the windows without scraping the brick on the way up.

What is that hook above the top window for?

That is the hijsbalk, or hoist beam. Amsterdam still taxes furniture removal companies for using them, and plenty of residents use a hoist beam today exactly the way a 17th-century merchant would have: a rope, a pulley, and a piece of furniture swinging past three floors of glass.

Why are the houses so narrow?

Property tax in the Golden Age was calculated by canal frontage, the width of the house facing the water. Narrower meant cheaper. Builders responded by going deep instead of wide, which is why so many canal houses run 20 or 30 metres back from the street on a facade barely five metres across.

Which canals are worth walking to see this?

Herengracht, Keizersgracht, and Prinsengracht form the main ring and carry the widest range of facade styles, from plain step gables to elaborate 18th-century neck gables. An evening walk shows it best, when the lower windows light up and the leaning upper floors catch the last of the sky. Amsterdam by Night: Lighting the Water follows this stretch after dark, past working drawbridges and centuries of leaning brick.

Next time a facade looks crooked, it probably was built that way on purpose.