This paper discusses the packing of α-helices in proteins, particularly focusing on the coiled-coil structure. It is shown that when α-helices of the same sense pack together, they tend to do so at an angle of about 20°, leading to a coiled-coil structure. Two simple models—the two-strand rope and the three-strand rope—are described, and used to illustrate diffraction theory. These models are shown to produce a diffuse α-pattern, which may explain the diffraction patterns of certain proteins. The paper also discusses the diffraction patterns of α-keratin, which are explained by the coiled-coil structure. The main characteristics of the diffraction pattern include meridional arcs at spacings of about 5.15 and 1.5 Å, and a group of reflexions on and near the equator at spacings around 10 Å. The paper explains that the 5.15 Å reflexion on the meridian is due to the deformation of the α-helix into a coiled-coil, and that the energy involved in this deformation is likely to be small. The paper also discusses the Fourier transform of a coiled-coil, showing that the structure factor is non-zero only on the layer-lines. The major helix, which follows a gradual helix, has a radius r₀, a repeat distance P in the z direction, and a pitch angle α. The minor helix, which approximates the α-helix itself, has a distance r₁ from the axis of the minor helix. The structure is assumed to repeat after a distance c in the z direction, with the major helix making N₀ turns and the minor helix N₁ turns. The paper also discusses the effect of repeating sequences of residues on the diffraction pattern, and the effect of the helices not all running in the same direction. It concludes that the coiled-coil hypothesis explains the co-existence of the 5.1 and 1.5 Å reflexions on the meridian, and that the structure is likely to be based on α-helices arranged in a non-parallel manner for reasons of packing. The paper also discusses possible applications of the coiled-coil structure, including the possibility of explaining the structure of tropomyosin and α-keratin.This paper discusses the packing of α-helices in proteins, particularly focusing on the coiled-coil structure. It is shown that when α-helices of the same sense pack together, they tend to do so at an angle of about 20°, leading to a coiled-coil structure. Two simple models—the two-strand rope and the three-strand rope—are described, and used to illustrate diffraction theory. These models are shown to produce a diffuse α-pattern, which may explain the diffraction patterns of certain proteins. The paper also discusses the diffraction patterns of α-keratin, which are explained by the coiled-coil structure. The main characteristics of the diffraction pattern include meridional arcs at spacings of about 5.15 and 1.5 Å, and a group of reflexions on and near the equator at spacings around 10 Å. The paper explains that the 5.15 Å reflexion on the meridian is due to the deformation of the α-helix into a coiled-coil, and that the energy involved in this deformation is likely to be small. The paper also discusses the Fourier transform of a coiled-coil, showing that the structure factor is non-zero only on the layer-lines. The major helix, which follows a gradual helix, has a radius r₀, a repeat distance P in the z direction, and a pitch angle α. The minor helix, which approximates the α-helix itself, has a distance r₁ from the axis of the minor helix. The structure is assumed to repeat after a distance c in the z direction, with the major helix making N₀ turns and the minor helix N₁ turns. The paper also discusses the effect of repeating sequences of residues on the diffraction pattern, and the effect of the helices not all running in the same direction. It concludes that the coiled-coil hypothesis explains the co-existence of the 5.1 and 1.5 Å reflexions on the meridian, and that the structure is likely to be based on α-helices arranged in a non-parallel manner for reasons of packing. The paper also discusses possible applications of the coiled-coil structure, including the possibility of explaining the structure of tropomyosin and α-keratin.