Drawing an elipse

For want of something better, this was something that came up at the job site when we were going over ideas for a little decorative area in a house. We disn't need use it in the end.
It sometimes happens that I need to make an elipse for an arch over an opening or something. A while back I found this method in a book and promptly forgot the part that makes it most useful. It gives an elipse of exactly the height and width you need.
You start with the perpendicular height running up from the center of the width. Put a pin in at A to mark the height and use your tape measure hooked on the pin to find where a line equal to half of the width will intersect on both sides at B and C. You put two pins in there and loop a string around the three pins. Then you take out pin A and slide a pen around inside the loop keeping it pulled tight in a triangle of varrying dimensions to draw a perfect elipse the right width and height to fill the space. I haven't drawn one in here, the first one you draw is really good fun, give it a try.
I shall try to remember the method as a sailing boat with 1/2width written on the sail for when I need it again one day.