(1) For the unenlightened: I am sure he will explain these things sooner or later. While wishing for that, think of it this way: terza, the third position of the arm/sword, is when you hold the sword in front of you, true edge facing down. Seconda is when you turn your arm and hand so that the blade is horizontal and to your right, true edge facing right; quarta is on the opposite side; prima is when the hand is above your head and true edge facing up. So from the top, first, second, third, fourth follow each other in 90 degrees.
95) So that was the first part of the handling of the sword, which was about distances and body positions and such.
96) Next we'll talk about the second part, which is about defense. First we talk about actual defense, namely the guard, and then about offense.
97) The guard is a position of the body, as follows: the arm and sword on a straight line in the middle of the offendable parts of the body, and the body settled to the "ordinary pace". The idea is to hold the enemy at distance, but to be able to attack if he comes close.
98) Terza(1) is then a position that is exclusively a guard and nothing else. Do not hold the terza outside of the knee, but so that it properly is in line with the middle of the body both horizontally and vertically, and ready to attack and defend on all possible lines.
99) Prima, seconda and quarta are not guards(2), because they are not well-suited for seeking measure, as they uncover too much of the body and are not equally close to all places you potentially need to defend. They are a means of striking, not of guard.
100) There are three things that make striking a target difficult. First, being too far away from it. Second, if it is covered. Third, if there is something close that can come to its defense quickly.(3)
101) All these virtues are in our guard. By leaning away from the adversary and keeping weight on the left leg we create distance. By holding the sword in the middle of the body we cover most of the target areas, and what is not covered can be quickly made covered. Thus one walks safely to take well the tempo and measure, which thing is the ultimate perfection of the guard.
102) To me, it does not make sense to talk about changing from guard to guard. You either are on guard, or you are not.
103) Offense is the type of defense where I actually try to hit the opponent.
(2) Err. Right. Whatever...
(3) The target being able to move away quickly apparently does not count.