Hi all.
Thought i'd throw in a few ideas from the world of scuba diving in general and technical diving in particular. Some of these ideas should apply to the dry land aswell. The world of technical diving is somewhat specialized as we often stick our heads into places they probably do not belong, such as cramped spaces, wrecks, caves, etc. This means a few things to consider when it comes to placement of your knives. Note the plural there btw
First off. Knives at the inner side of the leg == crap. It was popular in the 60's and 70's adventure series and looked cool, but it's way to long off to be usable. Plus it can get entangled easily in seaweed, lines, nets, and any other obstruction. Also, if your in a cramped space and/or in a dry-suit, you might not be agile enough to get to the knife.
This means placements needs to be hip upwards, within easy reach from both hands. And that leads us to where to put the knives. Here, a knife thats behind me (on the back of the hip) means i will first off not be able to reach it easily if my arms are in any way obstructed. Secondly, i will probably not see if i drop it. It will simply not be there when i need it. It's discreet yes, but if you need to be discreet while moving around through populated areas, put it in the backpack instead and take it out when entering the woods. My preferred placement for the first knife is thus front left or right depending on if you want crossdraw or not. Remember, the knife must be equally easy to draw with left or right.
The second knife should be at the upper body. Preferably shoulder area. on my setup i have a backup knife on the left strap, just forward of the shoulder. This knife has a snap-lock so i can get it loose by grabbing and yanking it. But its extremely hard to shake loose. It's also a fairly small knife as this is a backup and not a main. The thing to remember here is why do you have your backup. In my world, it's always to be able to cut yourself loose from entanglement and/or webbing (ie. the gear). Thus it doesn't need to be a knife, it can also be a safety-cutter (like used in cars for the safety-belt) or similar. The reason it should be upper body is because thats where my arms are and the shoulder area can be accessed even if my arms are locked forward (ie. can't get to my hip). It's also close to my head which means i will be naturally careful with entangeling it.
Now. On to the third backup. And this is something you probably wont have in a scuba scenario but definately imho should have in a nature scenario. Lets now imagine that you loose your main knife. The backup is not meant for heavy carving or such, it may not even be a "proper" knife. So just pack in a 2nd main knife in the backpack. Simple and will be a lifesaver if you do not recover your main knife and should at least let you stay out for a bit more instead of trekking back to civilization muttering about that damn knife
As i said in the beginning of this somewhat long post, this is first and foremost applied to scubadiving but maybe some of the ideas can be applied to bushcraft aswell.
Better safe than sorry =)
/HuBBa