It is the space between the nside of the glass on one side to the inside of the glass on the other side. Total The frame will go between that. A typical brood comb is 1" thick and that leaves one bees space on each side of that (between 1/4 and 3/8" is a beespace) making it a total of 1" + 3/8" + 3/8" = 1 3/4". (or for those who are “English measurement challenged” that is 25.5mm + 9.5mm + 9.5mm = 44.5mm)
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