Locking your bike in Denmark

In the US, we're always told to make sure you lock your bike to something. Bike racks are designed so that you can lock through the frame of your bike. Here, it's totally different: everyone just locks their bike to itself. The bike racks where you can lock through the frame take up a lot of space, so they aren't as realistic when there are so many bikes. This is a typical Danish bike rack:

Also, people park their bikes everywhere: there often isn't an actual rack to use.

The bike pictured above is like most Danish bikes and has a built in lock on the rear wheel. You use a key to lock and unlock it. The lock keeps the back wheel from moving which in turn prevents someone from riding away on your bike. However, you can still move the bike if you pick up the rear and walk with it, wheeling it from the front. There's also nothing to stop someone from picking up your bike.

There are some problems with people going down the street and loading bikes into a van. However, it also seems like there's a lot of less malicious bike theft: there is an attitude that if someone hasn't locked their bike and you need to get somewhere, you can "borrow" their bike. The problem is that those "borrowed" bikes then get left somewhere that the owner doesn't know and it ends up sitting and being abandoned.