On the SPC chart, the area between the center line and the upper or lower control limit is divided into thirds. The third closest to the center line is referred to as zone C, also known as one sigma zone; the next third is zone B, also known as two sigma zone; and the third closest to the control limits is zone A, also known as three sigma zone.
Note
The sigma zone terminology is appropriate only if three sigma is used for the calculation of the control limits.