  Based on first-principles calculations and effective model analysis, all
possible straight nodal-line phonons in symmorphic space groups are uncovered.
A classification of two-band ${\textbf{k}}\cdot {\textbf{p}}$ models at an
arbitrary point on the nodal lines is carried out, and these nodal-line phonons
are found to locate along the high symmetry lines of the Brillouin zone.
According to our classification, there are two types of nodal-line phonons in
symmorphic space groups. The first type possesses a linear dispersion
perpendicular to the nodal lines, while the second type exhibits a quadratic
dispersion. The underlying mechanisms are revealed by symmetry analysis, and
the types of nodal-line phonons can be distinguished through the little group
of the high symmetry lines. Our work not only classifies nodal-line phonons but
also provides guidance to search for topological nodal lines in bosonic
systems.
