Base Extensions - Reduce Diameter - US Cup Reduce Diameter Base Extensions (RDM) or Skinny are the proper choice for use with Reduce Diameter Masts (RDM). We sell base extensions from: Chinook, Streamlined, Aeron, and Epic. The Base Extension is used to achieve proper tension and luff length of sails. It has a pulley system and cleat to downhaul and secure tension. US cup RDM extensions are compatible with US cup bases. They have a diameter of 1..5" (approx 38cm). RDM base extensions come in 2 constructions: Aluminum or Carbon; are available in lengths: Short (6cm), Medium (28cm) and Tall (48cm).