The 2:1 angle is the pointiest angle in a triangle that is twice as long as it is wide. Doreen Speckmann called this triangle "Spike." You can rotary cut "Spike" simply by cutting the appropriate-sized rectangle in half. However, to rotary cut its partner shapes, the triangle of equal height and base ("Peaky") and the 2:1 kite, you will need to make a 2:1 angle on your rotary ruler.

This is very much like drawing "Spike" on graph paper, so I'll start by showing that. On a sheet of graph paper, I have drawn a rectangle four inches high by two inches wide, following the heavier full-inch rulings of the paper. Next I drew the diagonal of this rectangle. This diagonal is a 2:1 angle. It goes vertically two inches for every one inch it goes horizontally. Notice how the angle line goes through the corner points of every second one-inch square. (And, in fact, it goes through the corner points of every second square of the smaller graph squares, as well.)

Now let's duplicate this angle on your rotary ruler. Turn your ruler face down so that the printed side is up. Tear off a long strip of frosty (not shiny) cellophane tape. Position this tape at an angle over the ruler so that it is centered over the following points (these are expressed as a lengthwise ruling followed by "/" and a crosswise ruling):
 

Using a fine-point permanent pen and a straight edge, draw a line on the tape, connecting these points. After it dries, rub the line to make sure it will not rub off on your fabric.

For the reverse of this angle, repeat the process from the other end of the ruler, using these points: 

Again, draw the permanent line connecting the points. Use your rotary cutter to trim the excess tape at the ends of the ruler.
 
Turn the ruler over so the printed side is face down again as usual. Now you are ready to cut any shape that requires a 2:1 angle!