Cats are by far the most common household pet in Switzerland, so much so that their presence has given rise to a unique urban design phenomenon: the cat ladder. Devoted owners have built a series of outdoor climbing aids to help their feline friends come and go as they please.

Some are charming in their ad-hoc nature (a slender ladder leaning on a mailbox for support or treads applied to a building's rain duct) while others are complex zig-zag structures permanently affixed to the side of a building. It's not all DIY either. Now you can find some more elaborate models—like an ornate wood spiral staircase—for purchase online.

Using the city of Bern as a case study, Brigitte Schuster documented these eclectic cat ladders in a forthcoming photo book, simply titled Swiss Cat Ladders. Combining her photographs with a series of diagrams and essays exploring the structures from a sociological and architectural point of view, Schuster captures the popular trend and raises questions about the human-feline relationship. As she writes in the introduction, these ladders "indicate a willingness to house the cats properly and respect the animals' needs."

These zig-zagged cat ladders have a built-in railing to protect cats from falling.

Here, the mailboxes are used as support for the cat ladder, which serves to extend the window ledge.

This one is a simple, pretty precarious-looking, bridge between the balcony and the tree.

This folding ladder—developed by Maike Franzen—consists of "treads" with holes cut through them, which the cat uses to go up or down. It was designed for temporary use and can be easily folded up and moved.

This wooden cat ladder grows side-by-side with the vines. Owners often put cat ladders in for safety, reasons—cats don't always jump and land on their feet. Here, the ladder provides an alternate path down for a cat eager to explore.

This ladder reaches up to the third floor of this apartment building and is one of the highest you'll find in Bern.

Attaching treads to this rain duct transformed it into a cat ladder.

The crème de la crème: this is a rare spiral staircase model that can be ordered on the internet.