Windshield wiper blades that fit hook arms often list various sizes for the hook: 1/2" & 3/4", 9x3 & 9x4, and 9mm. Are these all different sizes, or are any equivalent and, if so, which ones? More fundamentally, what do the sizes measure?
According to the Windshield Wiper Guide (which may not be the most credible), 9x4 is the same as 3/4", and 9x3 as 1/2". I was also able to put a blade that was supposed to fit both 3/4" and 1/2" hooks on a car that is listed as taking 9x3 blades.
From measuring a wiper blade, 9mm is the width of the hook, while 3/4" and 1/2" refer to the height of the hook.