How can you tell the difference between a 6 iron and a 9 iron?

The angle of the head is different. Some are for hitting the ball further, while some are to get more elevation on the ball.

Answer

Yup... the smaller the number is, the more closed the face is and the farther and lower it goes. The higher number is the more open the face is, the higher but not farther the ball goes. It all has to do with an angle.