To achieve good high-speed current collection, it is necessary to keep the contact wire geometry within defined limits. This is usually achieved by supporting the contact wire from a second wire known as the "" (in the US & Canada) or "catenary" (in the UK). This wire approximates the natural path of a wire strung between two points, a catenary curve, thus the use of "catenary" to describe this wire or sometimes the whole system. This wire is attached to the contact wire at regular intervals by vertical wires known as "droppers" or "drop wires". It is supported regularly at structures, by a pulley, link or clamp. The whole system is then subjected to mechanical tension.