While a cute title, the pillow we’re talking about is a pillow of water backed up due to large rocks & boulders, which are very common on many waters. Finding trout near such rock requires you to understand the water dynamic and movement about it. The first key is that it is a hard feature that water is forced into. It often results in water backing up and stalling in a small area just upstream, called a pillow. And any time you have a pocket of calm water that is shroud with overhead cover in the form of escape depth or broken water surface, you may find a large trout. That’s prime holding water. Understanding these features will also allow you to identify other boulders’ limitations to potential trout holding water.