I spent my first couple of years piecing with whatever spool happened to be sitting on the machine, which was usually a heavy all-purpose polyester left over from mending jeans. The blocks went together, so I figured thread was thread. Then I made a nine-patch sampler where every intersection sat up like a little pillow and no amount of pressing would flatten it. My seam allowance wasn't the problem. I'd sewn every one of those seams with a thread far thicker than the job needed, and all that extra bulk had to go somewhere.
The number on the spool runs backwards
Thread sold to quilters is almost always labeled by weight — 40wt, 50wt, 60wt — and that number works the opposite way from how most people read it. In the weight system, the number is how many kilometers of the thread it takes to add up to one kilogram. It takes more kilometers of a skinny thread to reach that kilogram, so a bigger number means a finer thread. A 60wt is finer than a 50wt, which is finer than a 40wt, which is finer still than the 28wt you'd use for decorative stitching.
Two other systems show up on spools made for industrial or garment sewing, and they run the intuitive direction. Tex is the weight in grams of 1,000 meters of thread, and denier is the weight in grams of 9,000 meters; in both of those, a higher number means a heavier thread. If you find yourself staring at a cone marked Tex 30 and wondering where it falls, the rough translation is that Tex numbers and weight numbers move in opposite directions, and Tex 30 is a good bit heavier than the 50wt most quilters piece with.
Ply count muddies things a little. Two threads can both be called 50wt and behave differently if one is a two-ply and the other a three-ply, because ply affects how round the thread sits and how much lint it sheds. That's worth knowing, but it isn't worth agonizing over — the weight number still gets you most of the way to the right spool.
What 50-weight actually buys you in a seam
Look at a four-patch intersection and count what's stacked there: four pieces of fabric, eight seam allowances, and a line of stitching running through all of it. Every strand of thread in that stack takes up room. Drop from a 40wt down to a 50wt across a whole quilt top and you're removing a small amount of bulk from every single seam, which is exactly where it compounds. My pillowy nine-patch got flat the second time around, and the only thing I changed was the spool.
The other payoff is accuracy. A thicker thread pushes the folded fabric slightly further over when you press a seam to the side, which nudges your finished piece a hair smaller than the math says. Quilters call that the turn of the cloth, and it's why two people can both sew a careful quarter inch and end up with blocks a sixteenth apart. Finer thread doesn't eliminate it, but it shrinks the error, and it does it on every seam without you having to think about it again.
Finer thread also means less lint in the machine. Cotton sheds, and a heavy cotton sheds more per seam than a fine one does. If you've never opened your bobbin case after a full day of piecing, do it once — what you find there is the same stuff that's been packing in around the tension discs.
Cotton, polyester, and the argument about which one cuts which
You'll hear that polyester thread saws through cotton fabric over time and should never be used in a quilt. That warning came from a real place: the coarse, strong polyester threads sold decades ago for garment and upholstery work genuinely were stronger than quilting cotton, and the idea behind it is sound — you want the thread to be the thing that gives way, not the fabric, because a popped seam is repairable and a torn patch isn't. But a modern fine polyester made for piecing is nothing like that old upholstery thread, and plenty of careful quilters use it without any trouble.
Thread strength isn't guesswork, either. Sewing thread has a published test standard — ASTM D204, Standard Test Methods for Sewing Threads — covering breaking force, loop and knot strength, elongation, shrinkage in dry heat and boiling water, and colorfastness to laundering. Those are measured properties, which is a useful thing to remember when a spool's marketing copy starts making claims. It also explains why shrinkage matters: a thread that shrinks at a different rate than your cotton will show up as puckering after the first wash, no matter how relaxed your stitching was.
My own rule is simple. I piece with 50wt cotton because it presses flat, it behaves predictably under a hot iron, and it shrinks at roughly the same rate as the fabric around it. I keep fine polyester on hand for anything that needs to stretch a little or resist abrasion. What I don't do anymore is piece with whatever heavy all-purpose spool is closest.
One neutral spool beats a drawer full of matched colors
For piecing, thread color matters far less than new quilters expect, because the stitching is buried inside a seam. A medium gray, a soft tan, or an ecru will disappear into most scrappy tops, and one large spool of a good neutral will carry you through several quilts. I keep three: gray for anything cool or mixed, tan for warm prints and reproductions, and a true white for pale backgrounds.
Color does start to matter in two situations. The first is very light solids, where a dark thread inside the seam can shadow through to the front once the quilt is pressed. The second is when seams get pressed open, since the stitching then sits right at the surface of the split and is much easier to see. In both cases, go lighter rather than darker — a thread slightly paler than the fabric hides better than one slightly deeper.
When I reach for something other than 50-weight
Fine silk or 80wt cotton for hand appliqué and English paper piecing, where the whole point is stitches that vanish. Forty-weight when I want machine quilting to show a little, since the same qualities that make a thread visible on the surface make it a poor choice buried in a seam. Heavier perle cotton for big stitch quilting, which needs a needle with an eye to match. And I stay away from cheap bargain-bin thread entirely, regardless of weight — the fuzzy, weak stuff that snaps mid-seam costs more in frustration than it ever saves.
A thread routine for a piecing session
Here's the whole thing in order. Start with a fresh 50wt cotton spool and a bobbin wound from the same thread. Put in a new needle sized to that thread, usually an 80/12 for ordinary quilting cotton. Sew a test seam on two scraps, press it, and measure the finished width before you touch the real blocks; if it's off, that's your quarter inch or your turn of the cloth talking, and you want to hear it now. Every few hours, pop out the bobbin case and brush the lint away. When a spool starts shedding fuzz or breaking on long seams, retire it instead of fighting it. None of that takes more than five minutes of a sewing day, and it removes a whole category of problems you'd otherwise spend the afternoon blaming on your seam allowance.