Bees will not build in a gap that is very specifically 6-9 mm in width (but they WILL build in larger or smaller spaces).

INTERESTING FACT

Bees will not build in a gap that is very specifically 6-9 mm in width (but they WILL build in larger or smaller spaces). This discovery, called "bee space", made modern beehives with removeable "frames" for honeycomb possible; otherwise, the bees would just cement everything together. Amazing WTF Facts