The 2:1 resonant TNOs are classified into four groups: (1) objects with a lightcurve from the literature (blue squares; see Section 4.1 for more details), (2) objects with a flat-to-moderate lightcurve amplitude from our survey (green triangles), (3) objects with a large amplitude from this survey (red diamonds), and (4) objects never observed for lightcurve study (black circles). Our survey is combined with published lightcurves of 2:1 TNOs to cover a range of inclination, eccentricity, and absolute magnitude. Note that the nine likely 2:1 TNOs mentioned in the Introduction are plotted. Orbital elements and absolute and visual magnitudes are from the Minor Planet Center (2022 February).