In Watson-Crick B-DNA (the most common physiological form), each complete right-handed helical turn contains approximately 10.4 base pairs (more precisely ~10.5 bp/turn). The helix pitch (height per turn) is ~3.4 nm, and the rise per base pair is ~0.34 nm. A-DNA has ~11 bp/turn; Z-DNA (left-handed) has ~12 bp/turn. Watson and Crick originally proposed exactly 10 bp/turn in their 1953 model; later X-ray studies refined this to 10.4.