Card Trick - a nine patch block

Perhaps the optical illusion quilt block that everyone knows.  This block uses four colours plus a background and depends on careful colour placement to work.
To draft this block first start with a nine patch grid.

In the centre square add diagonal lines to make a quarter square triangle.

Each of the four corner squares get turned into half square triangles

Each of the side squares are made from one half square triangle and two quarter square triangles.  To show this first draw a line diagonally to make the half square triangle side.

For the quarter square triangle side of the side squares place the ruler as though you are going to draw a line diagonally from corner to corner in the opposite direction but only draw the line from the outside edge to the diagonal line previously drawn.

The Quilt Codex shows how to make a single block here 

Jenny Doan from the Missouri Star Quilt Company makes a variation here that is an 82x93 inch quilt using jelly roll strips

Click here to go to the nine patch page