Aligning Layers in Photoshop
Sometimes you need to put two Photoshop layers exactly in register with one another. For instance, your scanner might not be big enough and you have to scan an image in pieces and join them back together in Photoshop seamlessly.
There are various ways to do this, such as making one layer semi transparent, for instance, but the way that works best for me is to set the top layer to 'difference' mode.
In difference mode, two identical images (or parts of an image) that are exactly aligned cancel each other out. So, if you put one layer in difference mode and drag it over the one below it with the mouse until they are close, you can tap the up/down/left/right arrow keys until the two images coincide exactly - or as nearly as possible.
When they are not perfectly aligned, you will see lots of shape edges and outlines. It becomes very obvious when they are spot on.