The big plate at the base is designed to mount on the deck of a boat. We have a smaller base to mount on our bridge so please make sure before you order which style fits your boat the best. We suggest using the Under Plate for Telescoping Mount for a more secure and stable installment – sold separately – The locking handle allows for up to 10 inches of height adjustment along the slotted tube. The top knuckle has secure detent adjustments to tilt your units for the best angle. The Telescoping Double Deck Mounts are CNC machined for precise fit and adjustments. Predrilled and tapped to fits most Hummingbird, Garmin and Lowrance electronics. (WILL NOT FIT HELIX 7 Model) Helix 7 adaptor plate sold separately Standard allen wrenches required Deck mounting hardware not included.