There are two basic guard types: standing guard, done by holding B; and crouching guard, done by holding D or DB.
There are three basic attack types: highs, mids, and lows. Highs are blocked by standing guard and crushed by crouching guard. Mids are blocked by standing guard, and lows are blocked by crouching guard. Lows hit vs standing guard, and mids hit vs crouching guard.
So to get past your opponent's guard, you have to use a combination of mids and lows. Mids are much stronger than lows—almost every low is punishable—so standing guard is the default defensive option.
High attacks don't get past guard at all, so they're only useful for interrupting an opponent who isn't guarding. Standard throws are highs that go through guard so they're most similar to lows.
If you don't do anything, you'll automatically block moves as if you were doing standing guard. This is known as neutral guard.
Neutral guard results in less pushback than standing guard. This reduction in pushback is so tiny that it's almost always insignificant. However, there are a few moves where it matters, the most notorious being Heihachi's f,F+2, which is much easier to punish after a neutral guard.
- After db+2 (+7), ws3 (i10) is blocked, implying it takes fewer than 3 frames.
- After db+3+4 (+4), 3 (i14) hits but df+2 (i15) is blocked, implying it takes 10 frames.
- After d+2 (+8), f+3+4 (i30) hits, implying it takes more than 22 frames.
Neutral guard is immediate from crouch, standing guard, and any blockstun that doesn't guard break.
There's no such thing as crouching neutral guard. There's no known difference between D and DB with respect to guard outside of a guardable recovery. However, in certain guard scenarios where a move forces crouch on block (e.g. Armor King f+4) a throw must be crouched under by pressing db. In other forced crouch scenarios, one can even block mids by holding D (e.g. Law f+1+2 block). This has been labelled as unintended (bug) and some instances have been patched out.
Some moves break guard when blocked. This doesn't damage any damage. It just stops the defender from guarding until they press B or DB again.
In a few cases, such as Lidia's HAE.1+2, this can lead to a combo, but usually it just punishes someone relying on neutral guard. For example, the second attack of King's df+1,2 hits after the first is blocked unless the defender holds B.
Low parry state lasts for 20 frames when done from standing but only 10 frames when done from crouch. As such, if a defender wants to, for example, low parry after being hit by a move that forces crouch, they ought to crouch cancel first.
When shifting from crouch to standing without a crouch cancel, the interaction with this window is similar to while standing where there is a lingering crouch. The low parry state lasts 10 frames plus however many frames D was released for.
As such, committing to a low parry by spamming DF has somewhat of a hole in it.
If you don't go straight from DF to n and instead have some time at f then that's extra time where you're not guarding at all.
Slower than crouching guard?
There's a myth that low parry is slower than crouching guard. This myth arises because it's not possible to low parry during a guardable recovery. For example, during the last 12 frames of a tech roll the defender can block lows but not parry them. Since blocking works in this situation but parrying doesn't, uninformed players assume low parry is slower.
There are many situations where it's possible to guard but not do anything else (such as low parry). This is because you haven't actually recovered yet, but the recovery permits guarding. Some examples:
- During the recovery of quickstand, b getup, and most tech recoveries.
- The hitstun of moves with a ballerina spin animation, such as Lee's ws2,4.
- After blocking or being hit by an attack in a string.
During a guardable hitstun, blocking lows with D on its own doesn't work; only DB is reliable. And for moves that are concerned with whether or not the defender is crouching, holding DB doesn't put you into crouch. This is notable against Law, whose b+2,1 is +16g, and he gets a launch with df+2 against an opponent holding DB.