Take a look at the hem of most t-shirts and you’ll see what’s called a coverstitch or coverhem. Most people assume that this is done by a serger. Unfortunately, that’s not always the case.
Some higher end sergers will indeed convert to a coverstitch machine. Or you can buy a stand alone coverstitch machine. I have the Brother 2340cv shown to the left, and it’s pretty kickass. I kept putting it off, thinking it was silly to have machine that does one little thing, but it really is useful if you sew a lot.
Unless your bursting at the seams with cash, though, I don’t think I would say such an investment was necessary. There happens to be a $5 Cheater McPeter way of doing it, which I employed for several years when I was without a coverstitch machine. Behold, the Twin Needle!!
It looks like two needles on one needle shaft, and that’s exactly what it is. You will insert the needle in your standard zig-zag sewing machine* like any other needle. You’ll also need to rig up a second spool of thread.
Once you’ve threaded both needles, you can sew as usual (though I like to take it slow). You might need some tension adjustment, as well. When you sew a regular straight stitch, the top of the stitching will be two parallel rows of straight stitching. However, the bottom (bobbin) thread will be a zig zag. It’s not quite as stretchy as a true coverstitch, but it looks almost identical.