Prologo Zero-II Saddle with Nack Rails: The Prologo Zero-II Saddle with Nack Rails is a great technical road saddle with a flat profile for short to medium rides suited to frequent shifts in the saddle & Nack carbon rails. Nack Rail Nano carbon fibre rail. Carbon fibre, Kevlar & aluminium filaments make up the seat-post clam area, treated with the production process called Braided Carbon Rail (BCR). This is done in order to obtain "the perfect fusion" & uniformity of the several type of fibres & materials. The result is a balance structure & resistant to penetration. Nack Rails Carbon fibre injection base Microfibre cover Flat profile for short to medium rides with frequent position shifts