|To select the images, Aperture uses Stacks. These are groups of photos that share a lot of similarities. You can make stacks yourself, or have them generated automatically. In the latter case, the stacks are made on the basis of time. It is assumed that you have made a series of pictures within a certain period of time. This is very useful for portrait photography. You can order the pictures yourself within the stack, with the best photo first. If you shut the stack, that photo will then remain visible. A number indicates how many photos are underneath. You could compare it to a pile of cards. You can use stars to sort the photos. The better the photo, the more stars it has. The combination of stacks and stars works very well.