In order to find out why double edged swords were preferred to single edged ones, I conducted some experiments with knives of different blade shapes. I found out something interesting, but I'll describe the experiments first:
A) thrusts into cardboard with 1) double edged dagger 2) single edged knife with point on the middle axis of the blade3) single edged knife with off axis (back) point; thrust were executed keeping a light grip wearing a glove to study the behavior of blades during penetration.
B) similar actions with wooden knife of the previous shapes against an hard surface
Conclusions:
1) the single edged blade with "middle point" generates great friction on the unedged side, this LIMITS PENETRATION and makes the weapon turn in the hand somewhat.
2) single edged blade with point located back rocks in the hand because the point is not on the same axis with the force you apply, the way to correctly stab with this blade shape is to thrust with a circular movement (up or down, left or right) that drives the point into the target, this requires good judging of distance or you'll hit to a side of the intended point.
C) the double edged symmetrical blade is the best penetrating of all when you limit yourself to stright stabs, penetrates more with less effort, the weapon does not torque in the hand.
Maybe this is a reason for the preference given to double edged swords, and it is a consequence of the preference given to the thrust.
