Anterior precordials & the transition zone: V3–V4

V3–V4 overlie the interventricular groove, the transition zone, where the R wave grows to roughly equal the S (R/S ≈ 1). This is the heart of normal R-wave progression.

• Transition zone (R = S) normally falls around V3–V4.
• R-wave progression: the R grows steadily from V1 to V6.
• Loss of progression (or an early/late transition) is a diagnostic clue later.