The ONE 2.0 sail requires no extension, and the ONE base can be used directly with the mast. The ONE sails from 2.5 - 4.0 use a combination of a size specific ONE extension and a ONE base. All ONE sails from 2.5 - 4.0 are provided with the correct ONE aluminium extension. The base is sold separately.
For easy rigging simply put the extension (or the base for the 2.0) into the mast and fit the base into the bottom. Then insert the mast up the sleeve of the sail, downhaul and go! No adjustments to the extension are required.