Cream together the margarine, shortening, and both sugars until light and fluffy.

Beat in the eggs and vanilla extract.

In a separate bowl, sift together the flour, cream of tartar, and baking soda.

Gradually add the dry ingredients to the creamed mixture, mixing until just combined.

The dough will be stiff.

Roll the dough into walnut-sized balls.

Dip a glass in sugar and use it to flatten the cookies.

Place the cookies on an ungreased baking sheet.

Bake in a preheated oven at 350°F (175°C) for 8-10 minutes, or until the edges are lightly golden.

Let the cookies cool on the baking sheet for a couple of minutes before transferring them to a wire rack to cool completely.