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In the Harry Potter books, Diffindo is called the 'Severing Charm' and it’s most commonly used to cut ropes and the like. However, in the last book Hermione uses it on Ron but misses, creating a 'slash in his jeans' and his knee gets cut, causing him to 'roar in pain'.

We've only seen Sectumsempra used once on screen when Harry directly uses it on Malfoy in the sixth book, but there it's mentioned that he is 'waving his wand wildly'. Wouldn't Diffindo, if used in such a fashion also cause a similar effect? Similarly, if it was able to cut Ron, it would also be able to, say, chop off an ear (George's)?

In that case, how are these two spells different, except for Sectumsempra seemingly used exclusively to hurt humans?

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    On the chance that it was entirely coincidental that you asked this question twenty minutes after I posted an answer to a related question, you may be interested in scifi.stackexchange.com/questions/106494/… – Alex Feb 17 at 6:51
  • @Alex That was actually the question which got me thinking about this. I didn't read your answer then. :P Thanks! – Naman Kumar Feb 17 at 6:54
  • Well my basic argument there was that Diffindo is a spell for use on objects while Sectumsempra is a spell for use on humans. – Alex Feb 17 at 6:58
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    Diffindo will cut only once and can be reversed using Reparo. Sectumsempra will cut forever unless intervened. It's counter is unknown (only known by Snape). – Baby Yoda Feb 17 at 7:29
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Diffindo is more easily countered.

While Diffindo and Sectumsempra both can be countered by other spells, Diffindo is far more easily countered. Reparo, a relatively common spell, can completely reverse its effect when used once.

“He pulled the old copy of Advanced Potion-Making out of his bag and tapped the cover with his wand, muttering, ‘Diffindo!’ The cover fell off. He did the same thing with the brand new book (Hermione looked scandalised). He then swapped the covers, tapped each and said, ‘Reparo!
- Harry Potter and the Half-Blood Prince, Chapter 11 (Hermione’s Helping Hand)

However, when Harry cast Sectumsempra on Draco, Snape had to use the counter-curse multiple times.

“Pushing Harry roughly aside, he knelt over Malfoy, drew his wand and traced it over the deep wounds Harry’s curse had made, muttering an incantation that sounded almost like song. The flow of blood seemed to ease; Snape wiped the residue from Malfoy’s face and repeated his spell. Now the wounds seemed to be knitting.”
- Harry Potter and the Half-Blood Prince, Chapter 24 (Sectumsempra)

After repeating the counter-curse three times, Snape still had to take Draco to the hospital wing, and that was with Harry not even knowing what the spell did and never having cast it before. Presumably it could be even more damaging when used by someone experienced, and may have more severe effects in that case.

“When Snape had performed his counter-curse for the third time, he half-lifted Malfoy into a standing position. ‘You need the hospital wing. There may be a certain amount of scarring, but if you take dittany immediately we might avoid even that … come …”
- Harry Potter and the Half-Blood Prince, Chapter 24 (Sectumsempra)

Additionally, it takes that specific counter-curse to even have a chance of healing the wounds Sectumsempra creates. When George’s ear is cut off, his mother (and the rest of the Order) can’t make it grow back because Sectumsempra is dark magic.

“Mrs Weasley had staunched his bleeding now, and by the lamplight Harry saw a clean, gaping hole where George’s ear had been.

‘How is he?’

Mrs Weasley looked round and said, ‘I can’t make it grow back, not when it’s been removed by Dark Magic. But it could have been so much worse … he’s alive.”
- Harry Potter and the Deathly Hallows, Chapter 5 (Fallen Warrior)

Though they both have the effect of cutting, Diffindo is more easily countered and doesn’t leave the cuts is makes as difficult to heal by the usual magical methods.

Difindo can be used painlessly.

As is seen when Hermione accidentally cuts Ron with it, Diffindo, the Severing Charm, can possibly cause pain when used on flesh.

D – Diffindo,’ she said, pointing her wand at Ron, who roared in pain as she slashed open the knee of his jeans, leaving a deep cut. ‘Oh, I’m so sorry, Ron, my hand’s shaking! Diffindo!’ The severed ropes fell away. Ron got to his feet, shaking his arms to regain feeling in them.”
- Harry Potter and the Deathly Hallows, Chapter 9 (A Place to Hide)

However, there’s a way to use it painlessly for amputation, since Newt Scamander mentions that owners of Crups are required by law to use a painless Severing Charm to remove their tail.

“Crup owners are legally obliged to remove the Crup’s tail with a painless Severing Charm while the Crup is six to eight weeks old, lest Muggles notice it.”
- Fantastic Beasts and Where to Find Them

There doesn’t seem to be any evidence that Sectumsempra can be used painlessly, though it’s impossible to rule it out entirely, it seems unlikely from what’s known about its effects.

Sectumsempra is Dark, Diffindo isn’t.

When George’s ear is cut off by Sectumsempra, his mother mentions that the spell used was Dark magic.

“Mrs Weasley looked round and said, ‘I can’t make it grow back, not when it’s been removed by Dark Magic. But it could have been so much worse … he’s alive.”
- Harry Potter and the Deathly Hallows, Chapter 5 (Fallen Warrior)

By contrast, Diffindo doesn’t seem to be classified as Dark magic. Ron uses it in fourth year to remove lace from the secondhand dress robes his mother bought him. It’s highly unlikely he’d use Dark magic, especially for such a relatively simple task (easily accomplished by scissors) in a fairly non-urgent situation.

“In a desperate attempt to make them look more manly, he used a Severing Charm on the ruff and cuffs. It worked fairly well; at least he was now lace-free, although he hadn’t done a very neat job, and the edges still looked depressingly frayed as they set off downstairs.”
- Harry Potter and the Goblet of Fire, Chapter 23 (The Yule Ball)

Sectumsempra therefore would be classified as Dark magic, while Diffindo would not. There are practical differences in how Dark magic affects what it’s used on, like its making the wounds it creates difficult to heal, which differentiates the two spells in their respective effects. Also, its classification as Dark magic would make certain wizards who’d willingly use Diffindo unwilling to use Sectumsempra, even for the same things they’d use Diffindo for, because they don’t want to use magic considered Dark.

  • Do we know that the spell that severed George's ear was Sectumsempra? – Zip Zap J Feb 19 at 21:02
  • @ZipZapJ Yes: Now Harry was flying alongside Snape on a broomstick through a clear dark night: He was accompanied by other hooded Death Eaters, and ahead were Lupin and a Harry who was really George... A Death Eater moved ahead of Snape and raised his wand, pointing it directly at Lupin’s back. “Sectumsempra!” shouted Snape. But the spell, intended for the Death Eater’s wand hand, missed and hit George instead – – Alex Feb 20 at 2:56
  • @Alex Excellent, thank you. Haven't read in a while. – Zip Zap J Feb 21 at 17:31
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Sectumsempra can't be healed in the normal way, because it is "Dark magic", as we see when George's ear was cut off.

  • Huh? Wasn't Snape able to heal Malfoy's injuries immediately after he got cursed? I just assumed that George's ear was unable to be fixed because only Snape knew the counter-curse... He says to Malfoy that he can even prevent scarring if treatment is received quickly. – Naman Kumar Feb 17 at 13:15
  • snape knew a lot of dark magic so he probably knew how to heal Sectumsempra but it doesnt say that there was no scarring – Antheloth Feb 17 at 17:17
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    Hmm no. @NamanKumar is more correct. I would also argue that healing is not dark magic. Whether he could more easily heal it is another matter entirely. But of course he created the spell so he would know more about it than anyone else, almost certainly (this doesn't mean there can't be exceptions but in this case there isn't). Even so there's still a difference between George and Draco: George's ear was completely cut off whereas Draco was slashed i.e. they're incomparable. Of course it could have killed Draco but then again so could have something happened to George in that regard. – Pryftan Feb 17 at 23:34
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    Since they said that George's ear was unable to be regrown, does that mean that if they had the ear they could attach it back? – Naman Kumar Feb 18 at 0:50
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Sectumsempra is just more powerful.

I found an interesting Quora post on this and it seems relevant enough so here goes...

Sectumsempra is extremely similar to a severing charm named Diffindo. Here's it's description.

With the Severing Charm, cutting or tearing objects is a simple matter of wand control. The spell can be quite precise in skilled hands, and the Severing Charm is widely used in a variety of wizarding trades. Useful as it is, this charm should be practiced with caution, as a careless swipe of the wand can cause injury.

Which is extremely similar to what we know of Sectumsempra and it's after-effects.

Blood spurted from Malfoy's face and chest as though he had been slashed with an invisible sword. He staggered backward and collapsed onto the waterlogged floor with a great splash, his wand falling from his limp right hand. Slipping and staggering, Harry got to his feet and plunged toward Malfoy, whose face was now shining scarlet, his white hands scrabbling at his blood-soaked chest.

(It should also be noted that the power of Sectumsempra can be controlled. When Severus used it on James, it only resulted in a little gash on his cheek, while Harry's wild cast almost resulted in Draco's death.)

Though the two are quite similar, they do have a few differences Sectumsempra is a curse, classified as dark magic, while Diffindo is a charm. Organs severed with Sectumsempra cannot be grown back. Those severed by Diffindo probably can They both have different wand movements.
- How did Snape invent the Sectumsempra in “Harry Potter”? - Quora

The post goes on to speculate that Snape perfected the Diffindo charm from "a charm used for cutting herbs" into "a curse capable of cutting through human flesh". So, from this we can see that Sectumsempra is (in essence) a refined Diffindo.

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    Why should Quora be considered a reputable source? – The Dark Lord Feb 23 at 21:23
  • because it's right (in this case) and provides a compressed version of the other answers and thus is just as reputable, i see no reason to believe that it is any less reputable then this site we have here. – Niffler Feb 23 at 22:02
-1

I would like to disagree with Bellatrix's answer (no disrespect intended):

While Diffindo and Sectumsempra both can be countered by other spells, Diffindo is far more easily countered. Reparo, a relatively common spell, can completely reverse its effect when used once.

I would argue that the fact that Reparo can reverse the effects of Diffindo but not Sectumsempra does not necessarily mean that the latter is more powerful. It may simply be that Reparo is only meant for repairing objects but not humans, as I argued in this answer.

However, when Harry cast Sectumsempra on Draco, Snape had to use the counter-curse multiple times.

This would make sense even if Sectumsempra was not inherently more powerful; slicing up a human can cause severe injury including a lot of blood loss. Snape trying multiple times only means that Draco was seriously wounded.

Additionally, it takes that specific counter-curse to even have a chance of healing the wounds Sectumsempra creates. When George’s ear is cut off, his mother (and the rest of the Order) can’t make it grow back because Sectumsempra is dark magic.

I would argue that we don't actually see evidence that there is some unique way to heal Sectumsempra that only Snape knows. This is what Snape actually did:

Pushing Harry roughly aside, he knelt over Malfoy, drew his wand, and traced it over the deep wounds Harry's curse had made, muttering an incantation that sounded almost like song. The flow of blood seemed to ease; Snape wiped the residue from Malfoy's face and repeated his spell. Now the wounds seemed to be knitting.

There doesn't seem to be anything here to indicate that this was not simply a standard way to heal major injuries. All it seems that Snape did was stop the bloodflow and seal the wounds. Those are things that would not be unique to Sectumsempra-inflicted injuries. It is possible that Snape was simply a highly skilled wizard with particular knowledge of healing spells.

As for George's ear, I would argue that we shouldn't interpret Mrs. Weasley's words too literally/precisely. The reason why she can't fix George's ear is not simply that Dark Magic was involved. If that was the case then Malfoy's wounds would have been unhealable as well. Or since Snape was able to heal Malfoy's wounds it would follow that he could also reattach George's ear. But I don't think that is very reasonable. Surely, Snape was not the single greatest wizard in the world. If Snape had managed to find away to heal such an injury there surely could have been someone else in the world who had managed it as well, and Mrs. Weasley should not have been so dismissive. Consider that Sectumsempra was not a secret spell that no one else knew about; as Lupin said:

Sectumsempra was always a specialty of Snape's.

This implies that while Snape was especially known for using it, others were well aware of it as well.

Additionally, what is actually the difference between Dark Magic and non-Dark magic? When you perform a healing spell does the body run a check to see whether the magic that cursed it was Dark or not and if it detects Darkness it decides not to heal? That seems quite far-fetched. Instead I would argue that the only thing that really matters is the destructiveness that occurred. Certain things can be healed because there was simply less damage. Other things cannot be healed because the damage is too great. In Malfoy's case the wounds were serious, but at the end of the day they were just cuts. Cuts can be repaired relatively easily. In George's case, on the other hand, the entire limb was severed from his body. That amount of destructiveness cannot be repaired by magic because it would require growing an entire new limb. When Mrs. Weasley said that she can't regrow it since it was removed by Dark Magic, we should interpret that as meaning that it can't be regrown because it was removed, and it happens to be that generally it is Dark Magic that removes limbs.

In other words, my point is that it wouldn't make a difference whether the magic that removed the limb was Dark or not; but any magic that would slice off a limb would probably be Dark.

However, there’s a way to use it painlessly for amputation, since Newt Scamander mentions that owners of Crups are required by law to use a painless Severing Charm to remove their tail.

This does not necessarily mean that there is a fundamental difference between Diffindo cutting and Sectumsempra cutting. It is possible that "a painless Severing Charm" means that you have do use an anesthesia charm beforehand, or that simply Crups don't have much feeling in their tails so cutting them would be painless regardless.

By contrast, Diffindo doesn’t seem to be classified as Dark magic. Ron uses it in fourth year to remove lace from the secondhand dress robes his mother bought him. It’s highly unlikely he’d use Dark magic, especially for such a relatively simple task (easily accomplished by scissors) in a fairly non-urgent situation.

I would argue that this is not really a relevant distinction. Magic would be Dark or not Dark dependent on what the magic does. If two spells did the same thing then they would either be both Dark or both not Dark. The fact that Diffindo is not Dark should only indicate that it is not the same spell as Sectumsempra which is Dark; it doesn't tell us what the difference is, though.

So what is actually the difference between Diffindo and Sectumsempra?

I would argue, as I discussed at length in this answer, that there are fundamentally different classes of spells. Some spells are designed for human use and some spells are designed for object use. Diffindo and Sectumsempra both create cutting effects. I would argue that the difference between them is simply that Diffindo is designed for cutting objects and Sectumsempra is designed for cutting humans. That is why Sectumsempra is Dark and Diffindo is not, and Diffindo is more easily reversed.

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