Ultratuts | Blog

The behavior of some tools changes when you add one or more modifier
keys (Ô, Shift, and Option for the Mac; Ctrl, Shift, and Alt for Windows).
As an example of how modifier keys can affect tool behavior, consider the
Rectangular Marquee and Elliptical Marquee tools :

• Hold down the Shift key while dragging. Normally the marquee selection
  tools are freeform — you drag however you like. When you hold
  down the Shift key while dragging, on the other hand, you constrain the
  proportions of the selection to a square or circle (rather than a rectangle
  or ellipse).

• Hold down the Option/Alt key while dragging. When you hold down
  the Option/Alt key while dragging a marquee selection tool, the selection
  is centered on the point where you first clicked. Rather than being a
  corner of a selection, that starting point is the center of the selection.

•  Hold down the Shift and Option/Alt keys while dragging. You can
  select from the center while constraining proportions by using the Shift
  and Option/Alt keys together.