Analysis of depth-migrated gathers is the basis of most interval velocity estimation techniques. These gathers, known as common image gathers (CIGs), should consist of flat events when the velocity used for the migration is correct (Faye and Jeannot, 1986; Al-Yahya, 1989). As a result of this assumption, interval velocity analysis has become a procedure of flattening events in CIGs. A CIG is collected at a specific surface location. Each trace in the CIG represents a sum of many input traces, after being transferred from time to depth by a 3-D prestack depth migration. The vertical axis of the CIG is given in depth units, and the horizontal axis is referred as offset. This short note defines the offset axis and its effect on interval velocity analysis. I contend that the commonly used definition, which orders the migrated traces according to their original source-receiver offset, can limit accurate analysis. A different CIG offset definition (Reshef, 1997) is used for comparison. The sensitivity of the different CIGs to velocity errors is analyzed, and I attempt to answer the question of where to apply the velocity correction, relative to the analysis location. This study was carried out under the assumption that the sub-surface structure is complex and justifies the use of 3-D prestack depth migration. Only kinematic aspects are addressed here.