  Different atomistic registry between the layers forming the inner and outer
nanotubes can form one-dimensional (1D) van der Waals (vdW) moir\'e
superlattices. Unlike the two-dimensional (2D) vdW moir\'e superlattices,
effects of 1D vdW moir\'e superlattices on electronic and optical properties in
1D moir\'e superlattices are not well understood, and they are often neglected.
In this Perspective, we summarize new experimental observations and theoretical
perspectives related to interlayer interactions in double-walled carbon
nanotubes (DWNTs), a representative 1D vdW moir\'e system. Our discussion will
focus on new optical features emerging from the interlayer electronic
interactions in DWNTs. Exciting correlated physics and exotic phases of matter
are anticipated to exist in 1D vdW moir\'e superlattices, analogous with those
discovered in the 2D vdW moir\'e superlattices. We further discuss the future
directions in probing and uncovering interesting physical phenomena in 1D
moir\'e superlattices.
