Photoshop Quick Tip: Marquee Modifiers.

Every tool in Photoshop has a default behavior.  Default is defined as "what this thing does when I left click, drag, and drop." When placing marquees or shapes on the canvas, we can change the default behavior of the tools on the fly by using keyboard shortcuts known as modifiers.  Usually, when we're talking about modifiers, we're talking about the SHIFT, CTRL, and ALT keys (SHIFT, COMMAND, OPTION for Mac), but in reality, they can be any keyboard shortcut.  Here's a few examples: 

 When creating a free form marquee, simply left click on the canvas and drag until the marquee is the size and shape you want.  This is the unmodified default behavior.

 If you would like to preserve symmetry by keeping your marquee at a 1:1 ratio, press and hold the SHIFT key after you start dragging.

 To make your marquee expand out from center, instead of from corner to corner, hold down the ALT key after you start dragging.  Hold down the SHIFT key in addition to the ALT key to maintain symmetry.

Ever start to create a marquee and then decide halfway through the drag that you're not happy with the location?  No problem!  Without releasing your left click, press and hold SPACE to move the marquee around the canvas without changing its shape or size.  Once you're happy with the new location, release the SPACE key and continue with the creation of your marquee.

The same modifiers will behave differently, depending on when you use them.  Notice how in the above examples, you're instructed to only press the modifiers after you've begun to drag?  There's a reason for that.  Pressing the modifiers before clicking causes completely different behavior.

If you already have an existing selection on the screen, you need to decide if you want to replace it with the new selection, add a chunk to it, or remove a chunk from it.  This can all be done with modifiers, as well.  (Note:  this also works with the shapes tools).

• To replace an existing selection, do nothing.  Create it as instructed above.
• To add to an existing selection, hold down the SHIFT key before clicking and dragging.  Once you've began dragging, you can then continue to hold SHIFT if you want the addition to remain symmetrical.  If you want it to revert to free form, release SHIFT once you've started dragging.  You can also add ALT into the mix if you want to expand from center.
• To remove portions from an existing selection without destroying the whole thing, hold down the ALT key before clicking and dragging.   Then, just like above, either press or release SHIFT and ALT in combination to achieve the desired results.

Happy Photoshopping!