These are the rules of competitive single canne. The original rulebook can be found in french here. The rules as presented here are not coming from a translation of this rule-book. Instead, I made them after spending a few afternoons with Xavier Lejeune (head of the French Apache club), and together writing down the most important rules. If anything is missing or imprecise, it is certainly my fault, not Xavier's.
The rules you see here were introduced in 2018. Before the rules were somewhat more strict, esepecially the rule about the valid lunge. You can check the previous, stricter rules here.
An attack is valid, if
it is preceded by a valid armé movement (valid armé movements are different between each attack, describing each attack here is too complicated, therefore I only enlist them later. It is best to learn the attacks from personal teaching or a video)
the armé movement and the hit together form a continuous movement, without any halt
during the armé movement the hand is behind the so-called reference-plane. The reference-plane is an imaginary plane, which passes through the attacker's spine, and perpendicular to the plane drawn between the two fighters' spine
at the moment of the hit the arm, and the stick forms one line.
So all in all the rules above says that during a hit you are allowed to diverge from the "ideal" plane. There is even a rule which tells you the latest time you can start such a divergence during the hit. If you diverge from the "ideal" plane during a hit, you have to start that diverging motion at last at the reference plane. So you can hit a lateral so that in the first part until you reach the reference plane your arm and the stick is parallel to the ground, and when you reach the reference plane, you start descending your stick. Such change of a direction is allowed only once during a hit. (This supplementary rule is the base of the hit which is called by the Slovenians 'deadly lateral').
So while in the ideal lunge the rear leg has to be straight, during the real lunge the leg does not necessarily has to be straight. However the hip has to be as low as it would be during and ideal lunge.
during hitting the head or the abdomen it is not allowed to go down to a lunge (or even to a half-lunge)
A few things which are allowed (not forbidden by the rules):
The priority-rule (or parry and ripost rule) helps to decide what happens when two fighters hit more or less simultaneously. More or less simultaneous means: one starts an attack, and the other also starts an attack before the first finishes his/her attack, and both fighters finish their attack, and both fighters manage to hit a target.
To put the priority-rule very simply: the one who started first has priority and if one was doing and immediate ripost, that one has priority.
A fighter can gain priority by one of the following ways:
Starts an armé movement, while her opponent does not attack
Makes a ripost, which means defends with the stick, and with one continuous movement starts an attack on the same side where he defended
Makes a ripost with a dodge, which means dodges from her opponent's attack, and immediately attacks back.
A fighter can lose priority by one of the following ways:
Finishes his attack
Her opponent riposts to his attack (see point 2 above)
If during the fight fighter A has priority, and fighter B has no priority, and
As a consequence of the rule above, if both fighters start at exactly the same time (which is very rare), they both have priority. If at the end of them movement both fighters hit a valid target, none gets a point.
If one fighter gets a hit, she is forbidden to start a new attack, untils she manages to defend another attack of her opponent (either with the stick, or with dodging), or until a certain amount of time (about the time of an attack) passes by.
It is also worth noting, that the priorty rule does not forbid you to make counter-attacks (attacking during your oppnent's attack). To put it briefly, you can do counter-attacks, if you are carefull enough not to be hit.