Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 270

Here is an exercise to show you the essential functions of the Alignment tool :
• Open the image align.xcf in the SampleImages folder on the DVD.
• Then successively activate all layers in the Layers dialog and resize them
to the outline size of their image object with the Autocrop Layer function
( Layer > Autocrop Layer ).
• Select the Alignment tool in the Toolbox. Click on every image object in
align.xcf with the tool. Take note that the markers appear at the corners,
enclosing the frame. Alternatively, you can select the objects by using the
previously mentioned rubber band function.
• Activate the Background layer in the Layers dialog.
• Select Active layer from the Relative to drop-down menu in the Alignment
tool settings. Then align the image horizontally and vertically by clicking
the corresponding buttons in the tool settings.
• Set the Square layer to active in the Layers dialog. Select Active layer from
the Relative to drop-down menu. Using the tool, activate the layer with
the green triangle. Align it vertically at the top.
• Save your image.
It is not very likely that you will be using this tool for usual applications
in image editing or collages. The tool's strengths lie rather in creating logos,
graphics, and navigation buttons used for designing web pages. You will
appreciate the tool when positioning text layers in navigation buttons.
Figure 3.84
Adjusting the Square
layer in the image