Selection Type: rectangle (or square...doesn't matter, we'll be creating a custom selection)
Mode: Replace
Feather: 0
Anti-alias: Checked
What is anti-alias? Any pixel-drawn line or shape that is not perfectly straight (such as a circle) can appear jagged along the edge(s). Anti-alias creates the appearance of a smooth-edge by partially filling in pixels along the edge with semi-transparent pixels. Draw a circle shape without anti-alias checked and then draw another with anti-alias checked and you will see a good comparison. Note...normally for a square or rectangle you would want to UNcheck anti-alias to get a nice sharp edge...but, in this case, we will be rotating our pattern later and will end up with jagged edges if we don't use anti-alias! Planning ahead!!! ;-)
