I just googled and got this
http://gatewayquiltsnstuff.blogspot....-volume-6.html
I have a book that shows different blocks that make secondary blocks when sewn together. I think it is called Hidden Blocks. Any block that has some sort of shape in the corners of the block will give you a secondary block when joined together. If you get two straight sided mirrors and put them at a 90 degree angle at the corner you can see the extra block.
Good luck, you will have fun searching!