It depends ... there are two ways to mount a Cannon downrigger using our system: The UM6080 base matches Cannon’s Universal Mounting Base (which we commonly refer to as the 'dovetail' base) and you can bolt the two directly together. In this case, you would make use of our mount and the internal splines to adjust the rotational position of the downrigger. If you'd like to use the Cannon swivel base, then you also need to purchase the CN2207 Adapter plate, as the swivel base uses a larger hole pattern.