A pattern for a block will tell you the finished size and then give you instructions to make one and square it up to 1/2 inch more than the size it said you would make. This allows for the 1/4 inch seam on all sides to sew it into your quilt. This is called the unfinished block, because it has not been sewn into the quilt along with other blocks yet.
Once you have sewn all the blocks together, you lose that extra 1/4 inch on all sewn sides. The outside blocks will still have the extra 1/4 inch on any side that hasn't been sewn to another block. If 3 sides have been sewn, then only the remaining side will have the extra 1/4 inch. So, if you were making a 9 inch finished block, it would now measure 9" on the three of the sides, and the other side will measure 9 1/4". If you are looking at a corner block, then two of the sides would measure 9" and the other two will measure 9 1/4".
With something like a rail fence, you just want to make sure you start with squares that are all the same size. You will lose 1/2" when all the blocks are sewn together. The rail fence pattern is a very forgiving pattern and loved by quilters all around the world. So a square that starts out as 6.5", when sewn on all 4 sides, will end up 6 inches.
Last edited by Barb in Louisiana; 07-02-2017 at 06:00 AM.