Schematics showing both the two-elliptic-tube configuration and lower-order kink motions therein. Illustrated in (b) are two identical tubes that have elliptic cross sections and that are separated in the x direction. The y − z plane is identified as the tube plane, the meaning of which is clearer in the curved representation in panel a. This study examines only two tube orientations (panels c and d), with the one labeled “x-major” (“x-minor”) corresponding to the situation where the major (minor) axis of either tube is aligned with the x axis. The tube centers are placed symmetrically with respect to the y axis. Only axial fundamentals are considered, making it possible to examine the three-dimensional (3D) wave fields with a 2D approach. Lower-order kink motions are classified according to their internal flow fields in the apex plane (panels c or d); hence, such symbols as S​_x and A_y (see the text for more details).