A coupling nut is a long nut tapped from each side to meet in the middle, rather than all the way through in the same direction, used to connect two threaded rods end to end..
3/4"-10 x 5" Zinc Plated Grade 5 Steel Coarse Thread Carriage Bolts 20PK The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened..