Sidemounting cylinders comes from cave-diving in the UK. Caves here can often be a bit of a squeeze, and cave-diving grew out of caving.
Wriggling through a cave, explorers came to sumps and needed dive gear to allow them to
go further. With the need to transport this kit through small holes to reach the sumps, often limited space to kit up, and then needing to wriggle through small holes once in the sump, sidemounting was the natural solution.
There are many variations of rigging cylinders for sidemount, but the common objective is to keep the cylinders close in to a diver’s side and in the same plane, so that the diver can wriggle through flat cracks or turn on his side to negotiate vertical cracks.
Other considerations of the mounting are easy access to cylinder taps, protection of taps and regulators, because they are tucked beneath the diver’s arms, and easy removal and re-fit, so that divers can get through narrower holes.
On longer cave explorations, sidemount cylinders would be swapped with cylinders staged at planned points during the dive.

If the divers’ main supply was on their backs, as developed in the big Florida caves, the stage cylinders would often be hung from shoulder to hip. This is a bit like sidemounting, but the cylinder is usually higher up, with the tap and regulator further in front of the diver.
As with sidemounting, there are many variations in rigging stage cylinders (including sidemounting them), but the term “stage” has slipped into general use to describe the shoulder-to-hip fitting that provides the most convenient way for open-water divers to carry an additional cylinder or two of decompression gas.
In addition to numerous side and shoulder-to-hip variations, stage cylinders can also be carried across a diver’s chest or belly, across the backside (“butt”, in US parlance), towed from a hip or towed from a crotch-strap.

When we describe divers as being “sidemount”, we usually mean that their primary gas supply is carried sidemounted instead of on their back.
If their primary gas supply is on their backs, with additional travel or decompression gas carried on their sides, these are stage cylinders.

The most common rigging of sidemount cylinders is with the bottom clip attached to the side of a butt-plate or tail. This is a device that fits onto the crotch-strap of a diver’s harness
and provides clip points to the side of each buttock. Such clip points are usually easy to reach, but much lower down than they would be on a waist-strap.
At the top, the neck of the cylinder is wrapped within a loop of a bungee that stretches from the back of the diver’s harness to a shoulder D-ring, so it can be slid forward or back conveniently.
Associated regulators can then be either held on necklaces or clipped to shoulder D-rings.
With sidemounting being used as a diver’s primary supply, it’s up to the diver to swap between regulators regularly in order to balance the gas consumed (and hence remaining) between the two cylinders.
Gauges on short hoses can be set to poke straight up, so that they are close to the diver and easily reachable.

The importance of getting the bungee for the top of the cylinder at the right tension was demonstrated to me the hard way when I tried sidemount diving for the first time with instructor Martin Robson at last year’s Vobster TekCamp.
The harness, especially the bungees, was a bit on the small side. So small that, once I was kitted-up, fitting the cylinder necks beneath the bungees was extremely tight. I needed Martin’s help to get the second cylinder in place.
We agreed that they needed to be a few inches looser, but more bungee was not immediately to hand, and with time pressing we decided to get on with the dive. I had guessed my weighting right for the rig and, with neutral buoyancy, trim was spot on, with the cylinders nicely pulled in.
I was immediately comfortable with the rig, and could get on with what I was there for, to take photographs and enjoy the dive.
All went well until one of my fins worked loose. On a warm day and a short dive I was wearing hiking socks rather than my usual Weezle bootees beneath my drysuit.
With 12-litre cylinders pulled in tight against both sides, I couldn’t bend far enough or reach round them to get to my own feet and pull the strap back on.
Martin was busy sorting out something with the other diver. When he had finished laughing, I suffered the embarrassment of support diver Jason Brown being my Prince Charming.

Divers with stage cylinders will usually already have a twin-set on their back. The usual rig
is for a point just below the middle of the stage cylinder to be clipped to a D-ring on a diver’s waist-strap, then the neck of the cylinder clipped to a shoulder D-ring.
The trick with the waist D-ring is to get it as far back as possible, but still in a position where it can be easily reached to remove and re-fit the stage cylinder.
Most divers like to have the cylinder neck clip fairly tight to their chest, to stop the cylinder and reg hanging out and clanging along a wreck.
Because stage cylinders usually carry decompression gas, marking them up clearly is extremely important, especially when more than one is carried.
There are two common conventions for this. Most training agencies mark both the mix and the maximum operating depth (MOD) of the gas on a stage cylinder. But GUE (the DIR or Doing It Right) agency, marks only the MOD, on the basis that a diver doesn’t need to be confused by any other information when all gas mixes are standardised.
One benefit on which all agree is having the diver’s name or initials on the cylinder to allow easy identification.
While sidemount cylinders will be breathed throughout most of a dive, stage cylinders are breathed from only during the stage of the dive for which the gas is needed. So rather than securing stage regulators to a necklace, they are usually bungeed back along the side of the cylinder, where there is no temptation to breathe off them by mistake.

There are two common reasons for removing sidemount or stage cylinders while in the
water – temporarily to fit through a small hole, or completely to stage them and come back to them later, as when leaving decompression gas behind and collecting it on the way home.
For temporary removal, a diver may not need to remove the cylinder completely. Sometimes it’s enough to undo the lower clip and swing the cylinder forward, leaving the neck attached.
For longer removal, whether you remove the neck or lower clip of a cylinder first is down to personal preference.
When refitting, some divers like to fit the neck first and then swing the lower clip back into place. Others like to do the lower clip first and then swing the neck into place.
The only consideration besides convenience is that when you’re breathing off a cylinder at the time it’s usually best to unclip the lower clip first when removing the cylinder and to clip the neck in first when replacing the cylinder.
When a cylinder is removed completely and left, remember to attach it securely to something so that it can’t go off for a dive all by itself, and leave you missing both expensive equipment and gas when you return for it.

With sidemount diving, the answer for multiple cylinders is nearly always one left and one right. They will both be carrying the same gas and, between them, are a diver’s primary supply.
As with an independent twin-set, a diver observes a disciplined regulator-swapping regime to keep the gas supply balanced.
The main purpose of stage cylinders is to be either left where they will later be needed, or carried to where they will later be needed. Until that point, they are just baggage.
In most cases the stage cylinders a diver is carrying will each have a different gas, optimised for just part of the dive, with some overlap of their operating depth range to provide redundancy should one stage gas fail.
Here again there are two preferences – balancing the cylinders between both sides of a diver, or carrying them all on one side. GUE mandates all stage cylinders on a diver’s left, facilitated by stage cylinders being aluminium and therefore close to neutrally buoyant. Other agencies leave it up to the diver.
My preference is for all on the left, not because I am DIR, but because I can then hang my camera on the right without it clanging against stage cylinders.
With stage cylinders carried left and right, there is one convention that is as important as cylinder marking: the richer mix should always be on the right.
Other than convention, there is no particular reason for this. It could have grown the other way round. But now that we have a convention, bucking the system could easily cause confusion in an emergency.

Whether a sidemount or stage cylinder should be aluminium or steel is largely up to individual preference. However, there are a few considerations to bear in mind.
Steel cylinders are negatively buoyant, so removing them in the water is like taking lead off your weightbelt. On the other hand, they are usually more compact than an aluminium cylinder of similar capacity, and can be pumped to higher pressure.
If a sidemount or stage cylinder is going to be permanently attached, using a steel cylinder makes it part of your buoyancy system and therefore leaves less weight on your belt.
Steel stages or sidemounts can be awkward to balance if there is more than one of them and they’re all on the same side, so steel cylinders tend to be carried matched left and right.
Most divers hate carrying excess lead, so when a cylinder is going to be removed in the water, making it an aluminium one means that the user’s buoyancy barely changes.
Similarly, if a diver is carrying only one stage cylinder, or all cylinders are on one side, aluminium doesn’t create the imbalance that steel would in such situations.

I often see divers who, sidemount-diving or with stage cylinders, are unaware of what their cylinders are doing dangling below or floating behind them.
I’ve seen technical divers and photographers with near-perfect buoyancy control manoeuvring carefully into a reef to avoid touching anything, unaware that the base of their sidemount or stage cylinder is busily breaking bits off the coral out to one side.
It’s like hitting the reef with a hammer, and much more harmful than a fin-kick. At least a fin has a bit of give in it.
Configurations that are great for caves or open water may not be so good when close to a fragile environment. If I find that a sidemount is coming close to a reef, I may swap it to the other side, or swing it across my backside, to keep it away from where it can cause damage.

So far we have been talking open circuit, where the main gas is either back- or side-mounted and stage cylinders are used for decompression gas.
Rebreather divers are the exception. They carry their bail-out gas in any of the ways that open-circuit divers would carry stage cylinders – stage-rigged, sidemount, butt-mount or otherwise.
These cylinders may be referred to as stages, but at least one of them will be serving a different purpose with a primary role of bail-out should a rebreather fail. Because of this, what looks like a stage cylinder from a distance may well be filled with a bottom gas.
With these cylinders needed for bail-out, at least one will need to stay with the diver throughout the dive, and cannot be staged at convenient places for his return.

Should the gas in a stage or sidemount cylinder be turned off or left on when you aren’t breathing it
Leaving it on could result in gas sneaking out and not being there when you do want it. On the other hand, if there is no pressure inside
a regulator, the pressure differential from outside could be reversed and either damage the regulator or allow water to seep inside, both of which could make it unusable when needed.
Most divers like to pressurise a stage-cylinder regulator and then turn the cylinder loosely off.
Then, throughout the dive, they just turn it on and off occasionally to make sure that it stays pressurised. This way water is kept out, but the risk of gas sneaking out in quantities is removed.
The usual exception is rebreather bail-out cylinders. Here, because the gas may be needed at a moment’s notice, it’s usually a good idea to ensure that a cylinder with a bail-out gas appropriate for the depth is left turned on.

Sidemount is great for kitting up and de-kitting in the water or awkward spaces. Once in the water, it’s good for getting though holes or through confined spaces. One or both cylinders can be easily unclipped from the base and swung forward to make the diver’s profile even smaller for really tight spaces.
But when divers need to carry a deco gas as well as main gas, sidemounting the main gas can complicate things. Adding stage over sidemount cylinders does make a diver’s sides a bit crowded.
Unless you are at the extremes of cave-diving or wreck penetration, a conventional back-mounted twin-set will probably be both more comfortable and convenient for hanging stages on the sides.
An instructor who works across the Atlantic has noted that some extremely overweight divers are using sidemount because they are too obese to fit on boat kitting-up benches with cylinders behind them, and also too fat to perform shutdown drills on a conventional twin-set.

Stage cylinders are largely dictated by the dive plan rather than by the environment. If you
need to carry separate decompression gases, you will probably carry them as stage cylinders.

While the open-water technical-diving convention has evolved for bottom gas to be carried on the back with decompression gases on the sides as stages, it could have been the other way round.
Florida cave-diving practices found their way into open water in the form of technical diving and then back to the UK. But I have heard tales from before that time of groups of UK divers who did it the other way round.
This wasn’t widely known, because the popular wisdom of BSAC, PADI and other training agencies at that time was that any gas other than air would be sure to lead to instant death. So the technical divers simply kept quiet about it.
These divers would carry deco gas on their backs and bottom gas in stage cylinders. This way, when back at their decompression stops they could easily send the unwanted cylinders up on lifting bags.
Thanks to Phil Harrison, who I diverted from the BSAC stand to point my spare camera at me for many of the surface shots. Phil, you have a good eye for a picture.
All pictures were taken at Vobster TekCamp,,