I don't usually tack (baste) quilts using diagonal lines nowadays (as seen in the scrap quilt photo in my previous post). This method, using a slanting diagonal stitch on top and a horizontal stitch underneath, in lines radiating from the centre, was something I learned from Japanese quilt books, very like a tailor's padstich. There are some advantages over tacking in straight lines - the stitches hold the layers together quite firmly and there is little chance that a whole line of tacking is going to clash with where you want to quilt. It takes a bit longer to remove though!
When I laid the quilt flat yesterday, the borders definitely looked a bit wavy, like there was slightly too much fabric in them. This illusion was caused by having tacking the centre quite densely and also starting quilting in the centre (about a third of the centre panel is quilted). As I tacked the borders, the waviness started to disappear.
If I am only going to quilt straight lines, it can be as effective to tack fairly close along each line, and not bother to tack at right angles. That is what I did yesterday. It seems to hold the layers just as well. I try not to have the stitches going in and out of the fabric in exactly the same rhythm, as that makes the quilt seem wavy.
1 hour ago