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i was looking up the general page for magic last night on the wiki and when i came to the spells section it said something this: "Some spells (e.g. Levicorpus) are apparently designed to be used non-verbally. While most magic shown in the books requires the caster to use their voice, some do not (and this may depend on the witch or wizard)."

my question is, i always assumed that starting 6th year they were encouraged to do as many spells nonverbally as possible. obviously something like levicorpus would have to be cast non-verbally, but the bit about some spells needing the caster to use their voice made me confused. i always thought it was due to skill and how well a witch or wizard was at doing non-verbal spells on whether or not they could do it, or obviously if they had the attention to do it rather than shouting it out in haste, but in our universe are there some spells that absolutely need the voice? if so, how do we even know which ones those are?