(a,b) FFTEM textures and corresponding fast Fourier transform (FFT) patterns of (a) M1 with pitch ptb=8.05 nm and (b) M2 with ptb=9.3 nm, viewed in the planes parallel to the optic axis. The arrows in a point towards domain boundaries of average extension 26 nm, which are roughly perpendicular to the Ntb layers. Presence of domains is also revealed by a diffuse intensity pattern in FFT, marked by a white arrow in a. (c,d) FFTEM image of Bouligand arches in M1 formed as imprints of the oblique helicoidal structure onto the fracture plane that is (c) almost perpendicular and (d) almost parallel to the helicoid axis of Ntb. The insets show the corresponding schemes of Bouligand arches of two types in Ntb, calculated for (c) θ0=17°, ψ=5° and (d) θ0=20°, ψ=57°. Scale bars, 100 nm.