on the side of
Su asiento a bordo era del lado del pasillo.
His seat on board was on the aisle side.
Él tiende a estar del lado del grupo más débil.
He tends to side with the weaker group.
Él siempre estaba del lado del pueblo.
He was always on the people’s side.
Mira la ventana del lado del pasajero.
Look at the passenger side window.
Nosotros no los invitamos, deben ser del lado del novio.
We didn't invite them; they must be on the groom's side.
— 3 Idiots (2009)