GIMP#6 More Selective: Advanced use of selection tools (Gimp 2.8)
Notes: Gimp 2.8 has seven explicit selection tools: Rectangle Select, Ellipse Select, Lasso (Free Select), Select Contiguous by Color, Select by Color, Intelligent Scissors and Foreground Select, plus it is possible to select using the Paths tool.
Each new selection has a mode: Replace (new selection replaces existing selection), Add (new selection added to existing selection even if not overlapping), Subtract (new selection which covers existing selection will be subtracted) and Intersect (where new and existing selections overlap will be selected).
The Shift and Ctrl keys when held down before your start to select are used to temporarily switch modes (Replace no keys, Add Shift, Subtract Ctrl, Intersect Shift+Ctrl).
The Shift and Ctrl keys when held down after you start to select either a rectangle or ellipse are used to control shape and position. Shift will make the rectangle a square or the ellipse a circle. Ctrl will center the rectangle or ellipse on the starting point of your selection rather than that being the edge. Shift+Ctrl will do both at once, making a square or circle centered on the starting point.