Use the Marquee modifer keys

As you’re making selections with the Marquee tools, you can alter their behaviour with some quick key combinations. Get used to these and you can alter the shape and drawing behaviour of the tools as you’re merrily making selections. In the images below, we’ve shown what effect these modifier keys have on the Marquee tools. In each instance we’ve dragged the cursor out from the centre of the image and although we’ve used the Eliptical Marquee, the same rules apply to the Rectangle option.

Perfect shape
Hold down the Shift key to constrain any selections to a perfect square or circle. Wherever you click forms the top-left corner of the selection – the selection grows from this point.

In the middle
If you hold down the Option key (Mac) or Alt key (PC) while drawing a selection, the point where  you first clicked becomes the center of the selection.

Centred and restrained
By holding down Shift+Option (Mac) or Shift+Alt (PC), not only will your selection be centred form the point  you first clicked, it will also be contstrained to a perfect circle or square.