Joining "quilt as you go" blocks with sashing

We get lots of questions about our "quilt as you go" patterns. When people get to the step where we have you join the blocks with sashing, they tend to get concerned that we did our math wrong. It's not that it's impossible for us to have done the math wrong. That has been known to happen but in this case, we promise the math is right. :) Hopefully the following tutorial will help alleviate your fears in this regard.

To start, this is an example of one of our "quilt as you go" blankets. This is block blanket one, which is the background for Forest Friends, Safari Shindig, Tricks and Treats, Gobble 'till ya Wobble, and Joy to the World. But the method is the same for any blanket that uses block blanket 2 or the building blocks pattern.

For the purposes of the tutorial we are going to focus on the three blocks that make up the center of the quilt: 

The pattern will tell you to join the 15" x 7" yellow striped block to the 15" x 19" pink lattice block using a 15" x 2" piece of sashing. Then it has you join these blocks to the 22" x 26" blue block using a 2" x 26" piece of sashing. This is where people get worried, because they see that 7+19 = 26 and it seems like we forgot to account for the 2 inches of sashing. Well, worry not friends. I will now attempt to explain why this will all work out.

First, you will attach the 2" piece of sashing to the yellow striped block using a 1/2" seam allowance. When you flip the sashing down it will hang out over the edge of the yellow striped block by 1". (There will be 1 1/2 inches of sashing showing but 1/2 inch of it is sitting on top of the seam allowance.) Next, you will lay the yellow and white striped block on top of the pink lattice block (with right sides together) and join them with a 1/2" seam allowance. When you flip the yellow striped block up the total height will be 26". Behind the 1" piece of sashing that is showing there is 1" worth of seam allowances. 1/2" from the pink lattice block and 1/2" from the yellow striped block. Thus, the blocks are actually butted up next to each other behind the sashing and there was no gain or loss in the respective heights of the original blocks. 

Now there is no need to worry when attempting to join these two blocks to the 22" x 26" blue block using the 2" x 26" piece of sashing. 

The same theory as above applies here. When you sew the sashing to the blue block with a 1/2" seam allowance the total width will increase by 1". Then, when you attach the yellow striped/pink lattice block with another 1/2" seam allowance the total width will be 37 inches.