Its the vertical component of the antenna that does the groundwave radiating. The taller the better. The horizontal bit adds capacitance which makes the antenna electrically taller, So whether it be a T or inv L or '7' )in the case of laser), the vertical bit is all important.

Your clothes line antenna was just like a squashed inv L, worked but not very efficiently because it wasnt very high. There is an optimum V/H ratio for an L or T and a clothes line isnt it.