It's a double needle! What does this mean?

I bought a double needle this weekend. It looks like this:

You can get one at any fabric store. It costs about $3. It fits in any sewing machine. If you sew, do yourself a favor and go out and get one. It's pretty amazing.

Here are a few of the reasons why I love it:

• You can sew parallel lines without the seams getting sloppy.
• You don't have to do anything fancy with your machine -- just thread just like normal, but with two threads. 
• If you sew with knits, this needle will make your life easier. It makes seams look super pretty and it allows the knit to keep its stretch since the back side of the seam is a little like a zig zag.

So I decided to try it out on some new knits that I bought. Here are the leggings I whipped up for Jo.

 See the nice stitching under the waistband?