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From ABOUT QUILTING.COM Here is a very good way to check to see that you are actually sewing a 1/4" seam: Test Your Seam Allowance

Let's set up your sewing machine to sew an exact quarter inch seam, the standard seam allowance used for quilts. Once you've mastered a quarter inch seam, your blocks will have units with sharp points and crisp corners, and they'll go together easily when it's time to assemble the block or quilt.
Some patterns will tell you to sew a 'scant quarter inch seam allowance.' When you see the term used, make the seam allowance about a pencil-line's width narrower than 1/4" (a scant seam is sometimes necessary for patches to fit together just right, see page 5). Be sure to brush up on your rotary cutting skills and learn important pressing techniques before you begin.
Materials to Test Seam Allowances

• Rotary cutter, mat and 24-inch rotary ruler
• Iron and ironing board
• Sewing machine with a quarter-inch presser foot; use the standard foot if you do not have a quarter-inch foot
• 2 light fabric strips, each 2" x 4"
• 1 dark fabric strip, 2" x 4" [h=3]Let's Test the Seam Allowance[/h]
  1. Install a quarter-inch or standard presser foot on your sewing machine. If using a standard presser foot, try to determine which vertical groove on the machine's throat plate is equal to a 1/4" seam allowance.
  2. Align a a 2" x 4" light strip with a 2" x 4" dark strip, right sides together and edges carefully matched.
  3. Sew the patches together lengthwise, feeding them through the machine with the 1/4" presser foot's right edge aligned exactly with the right edge of the patches. For a standard foot, align edges with the mark for 1/4".
  4. Sew another light strip to the opposite edge of the dark strip.
  5. Press seams allowances toward the dark strip.
  6. Use a rotary ruler to measure the dark center strip. It should be exactly 1-1/2" wide along its entire length. The outer strips should be exactly 1-3/4" wide along their entire lengths.
  If the dimensions of your pieced unit are accurate -- congratulations! You're ready to sew a quilt.