The backset is the measurement from the edge of the door to the center of the bore hole. Most door handle sets feature a 2-3/8" or 2-3/4" backset.
To determine the backset on your door, measure the distance from the outer edge of the door, to the center, or highest point of the bore hole. If there is existing door hardware measure from the edge of the door to the center of the knob's backplate.
For more information about door hardware, visit out Door Hardware Buying Guide.