The usual normal S-T segment of the surface electrocardiogram (ECG) contributes little to the diagnostic procedure. When the S-T segment is too long or too short, or when it is displaced upward or downward, it is commonly abnormal. Under such circumstances, an S-T segment abnormality usually contributes considerable diagnostic information. When the Grant method of ECG interpretation is used, it is possible to perceive abnormalities of the S-T segment that may otherwise be ignored or misinterpreted. This paper describes the method of identification and the significance of primary and secondary S-T segment abnormalities. When a mean vector constructed for the S-T segment displacement seen in 12 ECG leads is relatively parallel with a mean vector representing the T wave, it is, with a few exceptions, part of the repolarization process and is therefore part of the T wave. This may be called a secondary S-T segment abnormality. When a mean vector constructed for the S-T segment displacement seen in 12 ECG leads is not relatively parallel with a mean vector representing the T wave, it is, with a few exceptions, not part of the repolarization process and is therefore not part of the T wave. This may be called a primary S-T segment abnormality.