6. The Tools in the Toolbox / 6.2 Tools for All Kind of Layers
The Crop tool is used to cut an image or a document to a given size. Usually you first define the size you want to get. Furthermore you can optionally scale the result to that given size.
There are 3 possibly situations:
If all settings are done, you can select the area you want to keep by using a rubber box. Optionally you can turn on guide lines which help you to rotate the image composition.
By clicking Crop or double-clicking inside the selected area cropping is executed. In order to straighten the result, there are several ways to rotate the background:
If your picture is distorted perspectively, you can correct that by clicking and dragging on a corner point while holding the Ctrl key (Windows) or Cmd key (macOS).
The Crop tool is always changing the size of the whole document, while the lasso tool (see chapter 6.2.20) is changing the size of the active layer. If the active layer is the background layer (see chapter 2.11), the result of both tools is similar.
Tip: Typing Tab will switch the given height and width. Landscape will become portrait and vice versa.
Another tip: If you only want to crop with a certain ratio, the unit of the width and height has to be "Pixel" and Scale must be turned off.