Energy level diagram of E-type (left) and A-type (right) methanol transitions. Transitions from Slysh et al. (1999) belonging to the line series where absorption features are detected in W31C are also marked, at frequencies around 133 GHz. Known class I and II methanol masers transitions are marked with red and blue arrows, respectively. The black arrows represent the transitions that have no maser detection yet. The dotted arrows are transitions with detected absorption features in W31C. The green arrows indicate the thermal transitions of the A^± series observed in W3(OH).