Base Extensions - Standard Diameter US Cup Standard Diameter Base Extensions (STD) are the proper choice for use with Standard Diameter Masts (SDM). 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 base extensions are compatible with US cup bases. They have a diameter of 1.85" (approx 48cm). STD base extensions come in 2 constructions: Aluminum or Carbon; are available in lengths: Short (6cm), Medium (28cm) and Tall (48cm).