Below is a Bulb Scale which you can use to help identify the Shape Type of your bulb.  The code has a letter followed by a number.  The letter refers to the Shape Type and the number refers to the diameter of the bulb in eighths of an inch.  The standard household bulb is A-19: the A is the Shape Type and 19 is 19 x 1/8" or 2 3/8" in diameter.  The MR-12 bulb is the MR Shape Type and 12 is 12 x 1/8" or 1 1/2" in diameter. 

To identify Shape Type, click on the link to Shape Guide at the right.

The graphic below is meant as a guide and is not to scale.