Relationship between the ratio of the surface flux of the chromospheric Mg I emission line to the stellar bolometric luminosity, R′, and the Rossby number, N_R. Large circles represent PMS stars with narrow Ca II emission lines (<100 km s⁻¹), while squares represent PMS stars with broad Ca II emission lines (>100 km s⁻¹). The color of the large circles and squares indicates the amount of veiling measured in 8700– 8850 Å. Dots represent PMS stars with no significant detection of the Mg I emission line. ZAMS stars examined in Yamashita & Itoh (2022) are shown as cross symbols.