The narwhal tusk assembles its macroscopic helix from building blocks with opposing twists.

Narwhal tusks are among the most intriguing structures in biology due to their immense size and unique left-handed helix. Whether this unique macroscopic homochirality originates in the structural arrangement of tusk building blocks has remained elusive. Therefore, we combined multiple structural techniques to reveal the hierarchical structure of the narwhal tusk from its building blocks (mineralized collagen fibrils) up to the macroscopic left-hand helix. Thus, we show that the macroscopic helical structure is reproduced from the molecular-scale arrangement of mineralized collagen in a double helix with opposite chirality. While mineralized collagen predominantly aligns along the tusk long axis, systematic axial deviations build helical arrangements of fibrils. Cementum has a left-hand helix of collagen fibrils, while dentine contains a right-hand arrangement extending across annual growth layers that feature distinct biomineral properties. This discovery advances understanding of how biological tissues integrate multiscale structural patterns to achieve specific mechanical and biological functions.