Gitzo series numbers tell you the upper leg size.
Series 1 - Upper leg section of 24mm
Series 2 - Upper leg section of 28mm
Series 3 - Upper leg section of 32mm
Series 4 - Upper leg section of 37mm
Series 5 - Upper leg section of 41mm
I'm not sure if I am allowed to link to a thread on another forum, so here I've copied from "nrothschild" on another Nikon forum, as he has explained Mountaineer and Systematic very well:
"A Mountaineer:
1. Comes stock with a center column.
2. The post of the center column can be removed, allowing the tripod to go to ground level. A tripod with a permanently attached center column is very problematic because the minimum height is dictated by the length of the center column. Current GT series Gitzo Mountaineer center columns are detachable.
3. You have an additional option to remove the big top wing nut and some other internal parts, and replace them with a Markins TB-21 (for Series 2) or TB-30 (for Series 3) replacement plate. That replacement plate adds some stability because it replaces the "figure 8" configuration of the wing nut and platform, which is not the most stable possible geometry, and replaces it with a conical design, which is similar to a pyramid- very stable and likely the most stable possible physical geometry. I shoot a TB-20 on my older Series 2 Mountaineer and although I have not found it to outperform the stock center column arrangement in static image tests, I believe it to be more stable if under stress, such as a wind, or with a high center of gravity mount such as a gimbal- things I did not or could not properly test.
In addition, the Markins replacement plates allow the tripod to go to ground level, which is now a redundant feature but a critical practical feature for my older G1228 Mountaineer that does not have a removable center column. I was shooting at ground level when other stock Mountaineer owners were staring at their tripods trying to figure out how to get them below 15" or so
The Markins plate has a further advantage, very obscure and rarely discussed, except by me , because it is very arguably safer than the stock Mountaineer or the stock Systematic. It is probably equally as safe as the NSN safety plate on a Systematic (safety plate). The reason for this, which is important if you like to carry your tripod with camera and especially a heavy lens attached, over your shoulder, is the construction of the center column post. All center column posts have a threaded insert epoxied in place. *IF* that threaded insert were to separate from the tube, your payload will separate from the tripod. I have never heard of a case of this happening with a Gitzo but there are some well known and publicized cases floating about the net concerning some other, less expensive tripods. The benefit of the Markins plate is that these parts are bolted in place, with no adhesives in the mount chain.
Any time you throw a tripod over your shoulder you take a very slight risk and some risk is related to operator error- something is not battened down properly. The Markins plate removes a tiny element of risk but not all risk because you are still in the equation .
4. A Mountaineer weighs more than an equivalent systematic BUT it has a much narrower mount girth. For portability considerations, if you care more about weight, then the Systematic has an edge. If you care more about mount girth (you need to pack it inside a backpack or luggage) then the Mountaineer has a slight advantage- maybe; keep reading .
Systematic:
1. It has an inherently stable and simple top plate. It is arguably not quite as stable as a Mountaineer with a Markins plate because the Mountaineer has more robust bracing under the leg fittings. Does that have any material effect? Jrp suggests the Mountaineer with Markins plate (part of the MAJICA configuration sold in the ProShop) is somewhat more stable and illustrates his assertion here).
2. You can add a Center column as well as a leveler and some other more obscure tops to a Systematic. You can see now that these two models can actually be converted into each other, more or less. It is far easier to swap tops on a Systematic, though, than to install or remove a Markins plate from a Mountaineer.
3. A Systematic adds almost 2" per section of maximum height for the same equivalent folded length. That because more of the folded length is used to telescope the sections, where a Mountaineer has several additional inches occupied by the mount and wing nut that can not be telescoped.
An example is the GT3541L verses GT3541LS....
The GT3541L (4.7lbs) is 23.6" folded and extends to 59.1"
The GT3541LS (3.8lbs) is 21.7" folded and extends to 57.5"
The Systematic is only 1.6" shorter in max height but fully 1.9" shorter in folded length. If the Mountaineer were the same 21.7" in folded length it would only go to about 53" or so in height (with center column retracted).
4. The Systematic has a critical potential single point of failure, which is the single bolt retaining the top plate on the mount yoke. If that yoke loosens your payload can drop and there have been a couple of reports of this actually happening. The cause is not clearly understood. You should check the tightness of that bolt right out of the box before you even mount your lens (a tool is provided) and you should check it periodically, especially when new. One of our members thinks he improved things by cleaning some lube off the mount and plate and roughing up the smooth machining a bit with emery cloth. There is also an aftermarket plate (designed for the flat top only, not the center column attachment) that secures from the underside of the mount and makes it impossible to separate under any conceivable conditions short of the 3/8" mount stud snapping in half (unlikely)."
