L. L. Langstroth - Langstroth on the Hive and the Honey-Bee: A Bee Keeper's Manual

Langstroth revolutionized the beekeeping industry by using bee space in his top opened hive. In the summer of 1851 he found that, by leaving an even, approximately bee-sized space between the top of the frames holding the honeycomb and the flat coverboard lying above, he was able to quite easily remove the latter, which was normally well cemented to the frames with propolis making separation hard to achieve. Later he had the idea to use this discovery to make the frames themselves easily removable. He found that, if he left a small space (less than 1/4 inch or 6.4 mm) between the combs, or between the combs and the sides of his hives, the bees would fill it with propolis thus cementing the combs into the hive. On the other hand, when he left a larger space (more than 3/8 inch or 9.5 mm) the bees would fill it with comb which had a similar effect. (Summary from Wikipedia)