Two 45-90-45° prisms can be used together to form a periscope. This device allows you to look over walls or over the heads of people in a crowd.
Look at the image on the left. Think about how the image that you would see in the bottom prism would stay the right way up.
Periscopes are also used in submarines. They can remain hidden under water whilst the crew on board can see whether or not there are any ships above the waves through the periscope.