Consider the diving knife. Weapon, tool, safety aid, the primary role of a knife will be to cut through rope, line, net of weed in which the owner has become entangled. So the knife fulfills - or should fulfill - a very important saftey function. And, as with any equipment purchase, a little background knowledge can go a long way.

The key features of a knife are its blade, grip and sheath. Other considerations are whether one knife will be enough, where and how to mount the knife, whether a knife is the only cutting tool you need.

Blades are usually made from stainless steel. There's a compromise to be reached here. Real stainless steel doesn't rust, making it ideal for use in water. But it also blunts easily. And, because it fails to hold an edge well, it can be very difficult to cut with. So, most knives include carbon in their make-up, which helps the blade hold an edge better. The trade-off is that the more carbon is added, the more rusting occurs, and the more brittle the blade becomes. If you accept that the ability to cut through an entanglement quickly and easily is all-important, then a little rusting is a fair trade. It's preferable to buring air at depth while your shiny, unblemished knife bounces off whatever you're trying to free yourself from.

Typically, blades are manufactured from 304, 420 and 440 grade steel. The 304 grade stains least and is better for levering things, as it isn't as brittle as 420 and 440. However, it won't keep an edge as well. A good choice is 420, as it holds an edge well and doesn't rust dramatically. The 440 grade retains its edge very well, but requires a greater degree of care as corrosion forms quite quickly.

Titanium is an exotic alternative offered at premium prices. It does not rust, is about 30% lighter than a stainless steel equivalent, and maintains its edge very well. It is also anti-magnetic.

The blade normally has a serrated edge used for sawing (rope often found around wrecks and marinas, for example) and a plain edge which will slash easily through kelp stalks. Many knives feature a notched line cutter, which makes it easier to cut fishing line - it can be trapped in the groove and held under tension as you cut. Line can have a tendency to skip over the serrations on the main blade, making it difficult to sever otherwise.

Blades can have bowie-style pointed tips, or screwdriver, or chisel ends for light work. Some divers favour blunted ends, reasoning that they are less likely to puncture themselves.

Blade length also comes into play. A longer blade will cut more per pass. A smaller blade takes more work. Small knives are popular with cave and cavern divers, who would normally only expect to cut their own thin guide line. Blades typically offered run from around three to seven inches. Some feature a measuring scale, possibly to endorse the macho status of the owner (okay, I'll come clean: my knife's a whopper with a 7 in scale, but, to paraphrase Freud, there are times when a knife's just a knife - I really don't have a complex).

The knife grip is important. It may be a moulded grip or just an extension of the blade. Either way, it should be comfortable for your size of hand and you should try it with gloves if you intend to use them. The T-piece should be adequate to stop your hand sliding on to the blade, even when some force is applied. Knives are sometimes dug into sand or used for prying, and it's easy for your hand to slip. Some grips incorporate a hammer end. This can be used to rap on your tank to attract attention ('though it'll scratch the paint). Some moulded grips are removable to allow you to clean the tang. If you pack this with silicone when you first buy your knife, it will need only minor cleaning at the end of the year.

Sheaths vary mostly in how they actually retain the knife. Many modern knives use catches rather than the traditional over-handle loop. Some catches are easily tripped, resulting in the loss of the knife. I've lost two knives because of this while exiting the water. A well-designed clip is both easy to release one-handed, even with gloves on, yet hard to release by accident. Often, it is easier to replace the knife in its sheath by touch rather than by sight, so a positive lock that you can hear or feel helps to confirm proper replacement and avoid losses.

Straps should compensate for suit compression and changes in musculature as you move. An over-tight strap will cut off circulation. I leave my straps joined up because otherwise they have an annoying tendency to fall out of the sheath and get lost. I carry spare straps as standard.

There are many places - simply choose which works best for you personally. You should be able to reach your knife or knives easily with either hand, and it shouldn't encourage entanglement, but those are really the only criteria. Potential entanglements don't just include lines and cables around the dive site, but also your weight belt if it is dropped either by design or accident. Inside calf: popular with open water divers, but some knives can be too long for some people, and can dig into the ankle, or even pop the knife from its sheath with some models as you swim or kneel. This can be a problem for young divers. They need a knife just as an adult does; they dive in the same conditions and face the same hazards. I have found young divers to be very responsible and would have no concerns about them owning a knife. I would be very concerned if they did not.

Outside thigh: another option. Some suits have pockets here to retain the sheath and these can reduce the risk of the weight belt catching on the knife or between your leg and the sheath.

Console-mounted: an option with some smaller knives. Check that they don't interfere with electronic gauges or affect your compass. Make a compass check with both the knife installed, and removed.

Waist-mounting: seems to have fallen out of favour. Modern stab jackets would probably get in the way of most knives, and attaching the knife to the weight belt has always been controversial. This implies yet another dedicated waist belt.

Upper body: advanced divers involved in cave, cavern and wreck penetration diving often keep their knives here. Working in confined spaces where there may be loose rope, wire and cables, as well as the diver's own guide-line, presents a very real risk of serious entanglement. A knife on the lower leg not only provides an anchoring point that could easily snag debris, it could also be near impossible to locate. A diver in a tightly confined tunnel, corridor or fuselage, may not be able to reach back far enough. So, knives are often attached to BCD shoulder straps, inflator hoses or on chest straps, or secured to biceps or forearms. Two knives are standard issue with these kinds of divers.

Knives are not always appropriate. For working dives, a dive knife is no substitute for the proper tools. Another major problem is gill nets. Difficult to see, and draped loosely to they hang in folds, these nets have a track record for ensnaring divers. Ten years ago, Dave Eveson, an BSAC diving officer, discovered that scissors were far more effective cutting tools than dive knives and published his findings. Since then, diving companies have offered suitably marinised shears and slash knives developed specifically to counter this hazard. They are not a substitute for a knife - they should be carried in addition.

Some divers use curly cords to secure their knives to the sheath. The idea is that if you drop your knife, you won't lose it. Basically, your knife should be in one of three places: in its sheath, in your hand, or on the bottom alongside you. I am prejudiced against curly cords because a friend of mine was running when the knife fell out of its sheath. It boomeranged back, sliced through his 7 mm wetsuit and took out a tendon in his leg. It's your choice. I've made mine!

Some divers carry two knives. Often, one is a macho workhorse tool, while the back-up is often a small, very sharp blade kept purely as a defensive measure for cutting lines. This ensures that when you really need it, it will still be sharp.

Knives require little maintenance. Rinse and dry and sharpen them from time to time. When you travel by air, put them through the hold, otherwise security staff confiscate them.

By the way, does anyone know why in underwater films, the baddie always cuts the air hose? Why doesn't he just stab the hero?