main reasons for it failing. Especially in situations in which one or both of the participants are not well-trained in the technique, the procedure has been criticised for endangering two people instead of one. Many divers fit a second demand valve, often called an "octopus", to their diving regulators, for emergency use by another diver. Some go so far as to recognise that the out-of-air diver will want the demand valve they are breathing off at the time, as it is indisputably providing breathing gas, and almost certainly a gas suitable for the current depth. These practice "" in recognition of this tendency, and often use an extra-long hose (up to 7 ft long) to facilitate the procedure in tight spaces. These divers often wear the secondary demand valve on a "" which keeps it ready for immediate use under the diver's chin, where it can be retrieved for use without the use of the hands. Divers doing deep diving, cave or wreck penetration, or decompression stops may routinely carry a complete, independent bailout scuba set for their own or their buddy's emergency use. This is particularly prevalent in professional diving operations where it is often mandatory. Use of other emergency air sources such as octopus second stage, integrated DV/BC inflator units, bailout cylinders etc. also requires the learning of appropriate skills. These procedures are as complex as buddy breathing up to the point of sharing, and the fundamental difference is that the donor and recipient are not required to alternate breathing with a period when no air is available, which can be a big advantage. These alternatives to buddy breathing also require substantial learning and reinforcement to be reliable in a stressful situation. Avoidance In most cases the need for buddy-breathing or other gas related emergency is avoidable. The equipment is highly reliable when in good condition, and though occasionally breakdowns will occur without warning, in most cases user inspection and testing before the dive, combined with a reasonable planned maintenance schedule carried out by a competent person will pick up potential problems before they escalate to an emergency. Realistic gas planning and monitoring of the remaining gas supply in context of the time required to surface safely will prevent almost all out-of gas emergencies. Carrying a fully redundant emergency gas supply allows the diver to bail out independently of outside assistance if in spite of all precautions, an