This is the pattern I see a lot on the internet - the edging is all done in treble crochet. I've crocheted this up just to demonstrate what I mean. Look at the straight outer edges - they bulge a little in the middle. I'm not happy with this. When it is joined, it will bulge in an ugly way, and the bulges will be magnified at the outer edges of the blanket, giving a wavy, scalloped effect, rather than a straight edge.