The Rectangular Selection tool allows selection areas of a rectangular shapes to be defined.
Rectangular Selection Settings
The Feather slider sets the amount of feathering to create around the rendered selection area. Feathering softens the edge of the selection by the specified number of pixels. See also: Feather Selection.
The Antialiasing button toggles the rendering of aliased edges on the selection boundary. Antialiasing produces smoother curves and angles and is recommended for most uses.
The Fixed Size / Aspect Ratio option allows more controlled creation of the selection region. By setting the size to the desired amount, predetermined areas can be selected with ease. The area can also be specified as a percentage of the total layer.
Default Keyboard Shortcut: R
Ctrl is used to subtract from existing selections and constrain the origin of the selection such that the initial click determines the center of the selection. Simply release Ctrl prior to releasing Button1 if non constrained selection subtraction is required.
Alt is used to move an existing selection.
Shift is used to add to existing selections or to constrain the selection shape such that the height and width remain equal. If needs require addition without constraint, simply release Shift prior to releasing Button1.
Shift-Ctrl can be used to constrain the selection generation to equal height and width, as well as forcing the selection to generate from the point first clicked. It also serves the purpose of intersection selections.
When moving selections beyond the boundaries of the image, the selection area will be cropped to the image area. Selections cannot exist outside image areas. Selection movements and changes are kept in the undo history just in case.