Most geared bikes use a system of chain derailment to switch between gear ratios. What seems in essence like a common sense defying archaic technology is in fact a super-advanced synergy between chain, cassette and chain set- and the key players in that gear changing game are the front and rear derailleurs. Otherwise known as front and rear mechs, derailleurs are cable operated wonders of modern technology. Never before has shifting been so fast - never before have gear ratios been so wide. However, there's a host of standards, fittings, actuation ratios and drivetrain systems to consider; so choose carefully; and if possible always match your front and rear mech brand with that of the rest of your drivetrain.