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A note on foundation garments and how to properly wear your armour:
In order for armour to both protect you effectively and look good it needs to be well fit. Given the expense of custom fit harness its often surprising how little attention is paid to the arming garments beneath it that are so critical to a well functioning harness.
Unlike the old cartoons, armour is not worn by climbing in through a trapdoor ala onesie kid's pajamas. A variety of highly tailored and durable garments are worn on the body which contain eyelets or "arming points" that various components of a harness literally tie onto.
My advice, and plea, is simple. Please don't skimp out on your arming garments. While tailored pourpoinst, gambesons, and arming cottes are expensive if purchased commercially, they can often be made in a weekend or three and the experience and understanding gained will be invaluable in understanding the design and engineering of the metal skin you wish to comission. If your gambeson is poorly fit, your armour will also fit poorly. Well fit garments and armour should not result in you looking like the michelin man in a barrel. Also, avoid excessive padding when possible. Well-fit armour doesn't require as much padding as one might expect. Poorly fit armour requires excessive padding to protect not only from the blow but also from the armour itself as its driven into the body.
Think about it, is it really worth it to skimp on your gambeson and save $300 only to have your 3-6 *thousand* dollar harness fit like crap?
I've worked with a few commercial patterns out there to make my own gambesons and by far the best source I have found is Tasha Kelly's booklet on the Charles De Blois Pourpoint. The gambeson shown below is constructed from this pattern and booklet, with very limited previous sewing experience. Its a fantastic step-by-step guide that even a fellow who's never sewn on a button can figure out.
While we've made tremendous advances in materials science and in some cases using modern materials can yield very effective results, in most cases period solutions are still very effective, if not the most effective, solution for wearing armour. In general, avoid polyester and other synthetic fibers like the plague unless you enjoy sweating to death. They do not breathe and are very prone to causing the wearer to overheat, particularly in thicker heavier gambesons. I prefer linen fabric with cotton batting as the coolest and most cost-effective combination of materials for gambesons. Hemp, wool and bamboo can also be used to good effect, for varying cost. If you are in doubt as to the suitability of your material, set it on fire. If it burns, its good. If it melts, so will you.
That said, my armour is not meant to fit or function over hockey pads. Putting blue foam in an $800 helm will make this armourer die a little inside. Elbow cops should not fit on your knee, and knee cops should not fit on your head. Prior to beginning a comission with me please take the time to research and tailor a proper foundation for your harness.
As an example, to the left is a photo of what I wear underneath my own armour. I have cheated a bit by wearing a close-fit athletic T-shirt (modern "under-armor" as its namesake suggests, is fantastic) both for decency with regard to being in my underwear on the internet as well as simply providing a comfort and sweat-wicking layer. Ideally, a linen shirt would perform the same job in a period-appropriate fashion.
The vest is a pourpoint, which is a shaping garment that is intended to support the weight of the leg harness on the hips rather than on the shoulders. There are some modern solutions to support leg armour and while many are quite ineffective and uncomfortable a very effective option which is most common in the SCA is referred to as a "C-belt". In any case, the weight of the legs should hang off of the hips. If you raise your arms up you should not be goosed in the groin by your cuisses, which occurs if the leg harness is suspended off of the shoulders.
On the legs, over a pair of joined hose (chausses also work fine) is a pair of gamboissed cuises which are simply conical tubes that provide padding for the thigh and knees and tie, much as the leg harness will, to the pourpoint.
To the right is the the same garments as above but with my gambeson worn on top. Aside from an arming cap (padding for the head) this is exactly what is worn underneath my armour. Please note how the gambeson is *very* closely fit to the body, particularly in the arms. This is very important.
It is possible, and after the 14th century commonplace, to combine the functions of the pourpoint and the gambeson into a single garment. This is much trickier in terms of tailoring but if done properly is extremely effective. In modern parlance this is usually what is referred to when the term "arming cotte/coat" is used.
This particular gambeson uses a medium weight linen outer backed by a canvas weight linen core with a fine light lining on the interior. The gambeson has 2 layers of thin cotton/bamboo batting throughout, with 3 layers given to the collarbones, point of shoulder, sides of the elbow and over the hips. My next gambeson will use even less padding as I've found it unecessary for all but the most aggressive sparring.