# Why are TM_10 modes not possible in a non-magnetic waveguide?

In a hollow rectangular waveguide, we may either propagate transverse electric modes (TE$_{mn}$) where we have electric standing waves quantified by m and n in each direction, or transverse magnetic modes (TM$_{mn}$).

The lowest mode possible for TE is (10).

However, my physics notes say that for TM the (10) is not possible in a non-magnetic guide and that (11) is instead the lowest frequency for TM. Why is this the case? Would eg (20) be possible?