As a bit of background, you may occasionally hear someone refer to this as "Complete Difference of Charge". When this occurs, beware! You are in the presence of an Old Fart Herald. The analogous section in the Old Rules (tm) was so named, and even after some eight years old habits die hard. Like RfS X.1. Addition of Primary Charge discussed in Part IV, Difference of Primary Charges is an independent way of clearing a conflict. In other words, if X.2. applies you need not go any further.
This is an example of a rule which takes a lot of words for a simple concept: When comparing two simple coats, if all of the primary charges are substantially different then they are clear. There is no counting of CDs involved. "Argent, a lion sable" is clear of "Argent, a chevron sable" because these are both simple and the primary charges of a lion and a chevron are substantially different. Implementing this rule requires that you spot three things:
What is a substantial difference has also been discussed, but merits a bit more review. First off, it only refers to type. Posture, orientation, tincture, etc. don't enter into it. A substantial difference is one which is more than what would have been considered a cadency step in period. In practice, the vast majority of differences are substantial. These are all substantially different from one another:
lion, horse, wolf, fess, chevron, mullet, billet, fleur-de-lis, eagle, tree, rose, thistle, etc
It is actually harder to come up with combinations which are significantly but not substantially different, but here goes:
lion/tyger, horse/unicorn/pegasus, mullet/estoile/comet, chevron/chevron embattled/chevron engrailed/etc
The question of "what is simple" is defined in painful detail. This is what takes so much space in the rule. The wording used in the rule is a bit counter-intuitive, since it defines "charge" to mean a charge which can itself be charged (i.e. have a tertiary). This was done for the sake of brevity (a failed attempt, in my opinion) but at the sacrifice of clarity. For this discussion I am using a slightly different scheme. The following are defined as simple:
"Simple" as applied to X.2. does not apply anywhere else, and other definitions of "simple" in other rules don't apply here.
Now to apply these principles to some potential conflicts: "Argent, three billets sable" is clear of "Argent, two eagles and a crescent sable". They are both simple (with only a primary charge group) and the primary charges are all substantially different. Consider, however, "Argent, three billets sable" vs. "Argent, two eagles and a billet sable." These are not clear. While they are both simple, not all of the primary charges are substantially different.
"Vert, a fess between four towers Or" is clear of "Vert, a saltire between four towers Or". They are both simple (with a primary between identical secondaries) and the primaries (fess vs. saltire) are substantially different. "Vert, a fess between four towers Or" is not clear of "Vert, a fess engrailed between four towers Or" because here the change to the primary is not substantial, but merely significant. "Vert, a fess between four towers Or" is not clear of "Vert, a saltire between three towers and a hedgehog Or" because the second design is not simple, having non-identical secondaries.
Here's one that almost looks like it ought to be clear, but isn't: "Argent, two suns and an oak tree gules" vs. "Argent, two lions and a pine tree gules". While every charge is different, not every charge is substantially different. Conversely, however, "Argent, two oak trees and a sun gules" is clear of "Argent, two lions and a pine tree gules." (This one is really clearer if you sketch it out.) While there is still the oak tree/pine tree problem, now they are no longer in matching positions on the shield. (This interpretation is by no means clear from the rules, but there is an explicit Laurel precedent affirming it.)
Finally, consider the ever-popular pattern of "Per bend argent and azure, a mullet and a badger counterchanged" vs. "Per bend argent and azure, a portcullis and an escarbuncle counterchanged." They are both simple, with all primary charges substantially different, so they are clear. On the other hand, "Per bend argent and azure, a mullet and a badger counterchanged" vs. "Per bend argent and azure, an estoile and a trefoil counterchanged" are not clear, since a mullet and an estoile are not substantially different.
The permutations are endless. I've only touched upon a few of the possibilities. The official rules include other examples and a different approach to defining simple, so if I've been writing gibberish I encourage you to try the rules as well. The key idea is that when comparing two designs, if they are both simple (as defined for this rule) and the primary charges are all substantially different, they are clear and you need go no further.
Next time: why there is a fourth method of conflict-clearing, but no third. Extra credit goes to those who read the rules and figure it out ahead of time.
In terra pax,