1. I have n e v e r had a mouse in a living hive. 2. If I can't get my baby finger into the bottom board opening, no Maryland usa mouse of any dimensions has ever gotten in. 3. I always run the s h a l l o w side of the BB up. 4. Entrance reducers mean extra cost and extra work. 5. I have n e v e r used an entrance reducer in 35 consecutive years of beekeeping. Ciao - still learning, however. Jack the B-man John Iannuzzi PhD * "Singing masons building roofs 9772 Old Annapolis Rd * of gold." --Shakespeare Ellicott City MD 21042 usa * 20 Italian colonies [log in to unmask] * 3-1/2 decades in beedom