I credit your tumblr posts about armor for getting me back into drawing art again after a dry spell lasting several years. I'll scan it one of these days. I know how you feel about back sheaths for swords, but what about daggers/short swords? How long can it be?
There’s evidence from various Eastern cultures that short swords were worn across the back (sometimes two, crosswise) but my beef is with it appearing, and being justified for appearing, in Western “historical” settings when it’s just leaked over from fantasy.
I don’t mind it in fantasy if there’s a bit of realistic sense applied: the length of sword carried by human characters like Conan and Aragorn, or indeed non-human characters like Drizzt Do’Urden and Elric (who may have special dispensation due to being a sorcerer and not carrying a sword so much as a sword-shaped demon) is limited by them not having arms like Reed Richards or Elastigirl, after which all bets are off.
That’s why your question got a big grin and a fuller answer than just a list of blade lengths… :-D (And congrats on getting back into your art work again; I’m pleased to have had something to do with it!)
A quick and easy way to find out how long (or short) can be managed is to get a broomhandle, length of dowel, long ruler etc., and with enough of it in your “strong” hand to act as a sensibly-sized grip, reach back behind your head with your “weak” hand - it looks like you’re putting on deodorant - and brace it against the top of your strong-side shoulder. (If a friend or family member can do this, it’s far easier!) Now “draw”, and see how much “useful blade” comes up beyond the braced fingers representing the top of the scabbard.
Holding it firmly against your back represents a scabbard secured at top and bottom, and you’ll have to draw straight up until clear.
Now have your friend etc. hold the dowel, ruler, whatever just at shoulder height, or pass it through a loop of string - you can hold this from the front - to represent a scabbard secured only at the top. Note that IRL a scabbard like this would flap about in an annoying way. (However, it could also be one which has been released at the bottom by reaching back with the off hand to release a hook or strap - a usefully dramatic “threat gesture” like a gunfighter pulling his coat-tail back.) You can draw a slightly longer blade this way, but the increase is only a couple of inches at most, and you’re now bringing it horizontally forward past your own carotid and jugular…
I experimented with four swords from my own collection…
…and results are for my height, 5ft 10in / 178cm, build (overly chunky…) and arm-length 24in / 61cm palm to pit, YMMV:
(1) a Kris Cutlery “Bronze” - really brass, inaccurate enough to be a fantasy sword, but very pretty and, conveniently, the only one with a scabbard - blade 21in / 53.5cm…
(2) An Albion Swords Mainz gladius, 1st generation - blade 20in / 51cm…
(3) A Del Tin Del Tin 2140 Oakeshott type XIV (the original is in the Metropolitan Museum, New York) - blade 31.5in / 80cm…
(4) A Del Tin 2121 Oakeshott type X - blade 33in / 84cm…
(1) The “bronze” draws in both positions with only a couple of inches to spare, and occasionally “trips” on the lip at the top of the scabbard, risking a poke in the shoulder with a very sharp point.
(2) The gladius “draws” cleanly in both styles - who would have thought one inch would make such a difference? - but since it’s fully sharp, the horizontal draw past my neck is unnerving and a fumble without protection would be very dangerous. NB, the usual interpretation by military historians like H. Russell Robinson is that the Roman legionary not only wore his gladius at his hip on a belt or cross-strap, but usually his right hip for a same-side draw so as not to interfere with the position of his shield (one or two volleys of pila (javelins) would have been thrown at the enemy, then the shield wall would close up, then the swords came out.)
(3) and (4) - forget it. They’re both too long for me, and I suspect for most people my height who haven’t arms like gibbons. When my arm’s at full extension, there’s plenty of blade still undrawn, so it’s easy to see that an even longer blade (like the Braveheart claymore, for instance) wouldn’t draw that way either. Back-carry for these swords is as luggage, not for use, and as historical novelist Alfred Duggan suggested, it may be the only time and reason a sword was ever worn that way. As mentioned before, there’s no pictorial evidence of medieval European back-carry at all…
Back-sheathed daggers for over-shoulder draw? First thought is “Legolas did it in the LotR movies” and second thought is “I have no idea why, except that it looked cool and different.”
Third thought is, “no historical evidence or indeed reason.”
Daggers and knives, especially if fancy but even if just functional, usually went openly on the belt (see any amount of historical art for details of where and how) or secretly, though how “secret” such common practice was, I can’t say :-) up a sleeve, down a boot or in other concealed places like on a string round the neck. These would all be weapons for deliberate assassination or surprise back-ups of last resort - in both instances, wearing them openly on a back-scabbard defeats that purpose.
Tiny thin things like stilettos could also hide as part of rolled seams, punch-daggers as part of a belt-buckle, they could also be up inside the hilts of other weapons (Mughal and Rajput warriors seem to have loved this one) or even, if the steel is springy enough, curled under a hat-band. This was mentioned in James Clavell’s novel “Shogun”, and I’ve seen flat, slender blades that could do it. Not fighting knives, but for those times when any weapon is better than none. (The couple I handled were literally as sharp as razor-blades, and not much thicker.)
Over the shoulder for daggers is really just more fantasy - but at least they’re short enough to do it without difficulty, except for taking off your own ear or slicing open your own neck if you’re careless. But movie heroes never do that…
Hope this helps!