It is imperative (for road riding, at least) that the chain is sufficiently tight that it is impossible for it to derail from either the chainring or sprocket. This generally equates to "no visible slack". A derailed chain can cause a variety of undesirable consequences, such as a locked rear wheel or, worst of all, destruction of the frame if the chain becomes caught around the pedal spindle and pulls the rear triangle forwards. On a fixed-gear bicycle without hand brakes, even a relatively benign derailment means a total loss of braking ability. Tensioning aside, a chain is significantly less likely to derail if the chainline is accurate and the chain is a traditional "full bushing" type with limited lateral flexibility. Because the difference between a tight and a slack chain equates to only very minor elongation of the links, chain tension should be visually checked at least weekly, especially if the bicycle is ridden in wet or dirty conditions.