The center of the new pattern now contains both the new block and some of the old pattern.
The result is this new tessellating pattern.
If you are wondering how I knew this was going to happen, the answer is, I didn't. It came as a complete surprise to me. I started playing and there it was.
For more playing with tessellations go to My Posts by Subject in the sidebar on the right and click on Tessellation.
A Spoonful of Christmas 13, 14
2 hours ago