Hmmm
So listening to thelecture [we have not learnt group theory], the lecturer says. " we do not have plane centered tetragonal cells, because we can make a smaller unit cell which still have 90 degree angles but a smaller volume.
If that is the case surely you can make a smaller orthrombic unit cell with all angles still being 90 degrees, so why do we have 4 types of orthormbic cells