Left: R19 model; right: X23 model. Top: ℓ−v_LSR; bottom: X−Y. Points denote classical Cepheids with ages ≤100 Myr. Colors indicate spiral arms. R19 (left panel): Scutum–Centaurus–OSC arm (blue), Norma–outer arm (red), Perseus arm (black), Local Arm (cyan), Sagittarius–Carina arm (magenta). X23 (right panel): outer arm (red), Perseus arm (black), Local Arm (cyan), Sagittarius arm (magenta), Carina arm (dark green), Norma arm (orange), and Centaurus arm (yellow). Colored circles represent classical Cepheids (filled: with v_LSR; open: without), with colors indicating assigned spiral arms. For R19, the model v_LSR values follow the kinematic parameters of M. J. Reid et al. (2019); for X23, they are calculated from the spatial-arm loci using the Galactic rotation model adopted by M. J. Reid et al. (2019). The bar geometries shown in the two panels follow different literature-based representations: the relatively long bar adopted for the R19 four-arm model is taken from C. Wegg et al. (2015), whereas the shorter bar shown with the X23 multiple-arm model follows T. Hilmi et al. (2020), as adopted by X23.