There's usually more than a single 3/8ths inch (less than 0.5 cm) bee-space between the bottom of the lower edge of the lowest frames and the upper surface of the bottom board. My commercially made hives have about twice that, at least so figure perhaps 1.25 cm? I could measure mine exactly, if needed. The lower hive boxes rest on an upstanding lip around three sides of the perimeter of the bottom board. My equipment is from Betterbee, which has excellent pictures in their on-line catalogue to use as examples.
A standard Langstroth box had 1/4" (6.4mm) space at the top and 1/8" (3.2mm) space at the bottom. A standard reversible bottom has 3/8" (9.5mm) space on one side and 3/4" (19mm) space on the other side. With the 3/4" space up you have approximately 7/8" of space from the bottom board to the bottom bars of the frames. With the 3/8" (9.5mm )space up you have approximately 1/2" (12.7mm) from the bottom board to the bottom bars of the frames. This works because of two things. One: Bees don't tend to crowd things all the way to the bottom board. Two: Bees don't tend to build comb where the entrance is. A normal bee space anywhere else would be between 1/4" (6.4mm) and 3/8" (9.5mm).
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