Let's practice with the blocks in the illustration.
Left, Sawtooth Star with 16-patch CenterThe bottom illustration of the Sawtooth Star block shows the design's four-patch grid, and it's easy to visualize dividing the grids again horizontally and vertically, placing the star tips, block corners and four small squares into grids of their own (4 grids across and 4 down).
Would it be possible to make this block in a 12-inch finished size?
Let's look at the smallest pieces of the grids first, the little squares at the block's center. If you visualize the divided-grid arrangement, you can see there are two small squares in each grid.
- 12" divided by 4 = 3" (finished) for each of the grids
- 3" divided by 2 = 1-1/2" (finished) for each of the two small squares in a grid, a size that's rotary cuttable.
If a finished size is rotary cuttable, the patch will still be rotary cuttable after adding the seam allowance.
Would this block work in a 10" finished size?
- 10" divided by 4 = 2-1/2" (finished) for each of the grids
- 2-1/2" divided by 2 = 1-1/4" (finished) for each of the small squares
The half square triangle units and plain corner squares are doable for any rotary cuttable size, so a 10" block is one of your choices.
Right, Bonnie Scotsman Quilt BlockBonnie Scotsman can be assembled in a couple of ways. You can make units that look just like the four grids, but the block's patchwork will flow a bit better if you extend the long, light bars on the top and right edges to touch the dark corner square, rather than piecing them separately to create a four-patch unit.
The grid not only helps you calculate sizes, it allows you to diagnose the block's structure. A huge percentage of our quilt blocks can be assembled in multiple ways, and examining grids lets you begin exploring the options. Take a look at the Bonnie Scotsman Quilt Block Pattern to see how I chose to make the block.