Bottom-up cables are generally used for code Zero's or staysails where the cable is enclosed in a pocket on the luff of the sail. These are straight luff, upwind sails with a relatively straight leech profile and therefore forestay tension is important. Therefore, the most common core material for bottom-up cables is PBO due to its excellent stiffness. However, for some cruising applications, K49 can provide a cost effective alternative. Bottom-up furling is less "torsion critical" than top-down furling as the sail material contributes to the torque transmission after the initial couple of wraps - cable stiffness is generally more important to torsional performance.