A pieced block comes off the machine close to square and almost never exactly square. Seams pull a thread one way, the iron nudges things the other, and what should be a 4-inch square measures 3 7/8 across the top and 4 1/16 down the side. Squaring up is the trim that fixes it: lay a ruler over the block, line it up on the size you want, and cut the excess off all four sides. Blocks that all measure the same go together into rows without easing or unpicking. Blocks that are each a little different fight you at every seam.
Power Tools with Thread trims a half-square triangle here, from an English trellis quilt, in four minutes. The bit worth staying for is the painter's tape: mask off the ruler lines you are not using and the one line you need stops vanishing into the grid.
Work out the unfinished size first
Every block has two sizes. The finished size is what it measures once it is sewn in, seam allowances buried. The unfinished size is what it measures right now, and that is the number you trim to. Unfinished is finished plus half an inch, because a quarter-inch seam allowance gets eaten off each of the four edges: a block meant to finish at 3 inches gets trimmed to 3.5. Most patterns hand you that number outright, so read it off the pattern before you pick up the cutter.
What the ruler has to have
A small square ruler beats a long rectangular one here. It covers the whole block and turns faster between cuts. An inch or so larger than your trim size is the sweet spot.
For half-square triangles, flying geese, and anything else built on a diagonal, it also has to carry a 45-degree line running corner to corner. That line is what you lay on the block's seam, and without it you are guessing. Plenty of cheap square rulers have edge markings and no diagonal, so check before you buy. Mat and cutter are the same ones you already use to cut quilt fabric.
Trim all four sides, not two
The tempting shortcut is to line the ruler up on two edges and cut the other two. It is how blocks end up lopsided. All the error goes to one corner, the diagonal drifts off centre, and on a half-square triangle the point stops landing where it should.
Centre the ruler instead. Put its 45-degree line on the block's diagonal seam, leave a whisker of fabric showing past two edges, cut those, then turn the block and take the same whisker off the remaining two.
What to do when the block comes out too small
You go to trim to 3.5 inches and the block measures 3 3/8. There is nothing there to cut. This is the one nobody answers.
Do not stretch it to fit. Press it again with a shot of starch - a block pressed while damp often gains an eighth once it dries flat. If it is still short, trim the whole batch down to the smallest. The real fix is upstream: your seam allowance is running wide, and a scant quarter-inch seam on the next batch gives that fabric back.
Common mistakes
The ruler slides mid-cut. A dull blade causes most of it - you press harder, the pressure shoves the ruler, the cut wanders. Change blades more often than feels reasonable.
Starting the blade on the fabric instead of behind it leaves a nick at the corner. And lifting the ruler to check a cut - move the fabric out from under the ruler instead, and leave the ruler where it sits.
Skipping the press. A seam allowance folded the wrong way reads as thicker than it is and measures wrong under the ruler. Press the seams and nest them at the intersections before anything touches a cutting mat.
Squared blocks are what the rest of the quilt stands on. Once your half-square triangles and flying geese units all measure the same, the rows go together fast, and adding the borders stops being the step where the top goes wavy.
