Notes

A nonverbal spell was a spell which was performed without the casting wizard or witch speaking the incantation aloud. Hogwarts School of Witchcraft and Wizardry began teaching nonverbal magic to students in their sixth year in Defence Against the Dark Arts, Charms, and Transfiguration; the students were expected to use spells nonverbally from then on

Performing spells non-verbally was very difficult and required a good deal of practice, as it required concentration and mental discipline alone. Harry Potter noted that several students in his class looked "like they had received an overdose in U-No-Poo", (a product of Weasley's Wizard Wheezes that caused constipation), while attempting nonverbal spells.[1]

Different wand-woods might also affect the performance of casting non-verbal magic. Many wands made from Dogwood were rather noisy, and refused to perform non-verbal magic, whereas wands of Pine were some of the most sensitive to non-verbal magic.[2] Willow wands possessed a well-founded reputation for enabling advanced, non-verbal magic, but of all woods, Alder was known as being the best suited for non-verbal magic.[2]

Some spells, such as Levicorpus, were easier to perform nonverbally than others. Most spells, however, seemed to be less effective than normal when the incantation was not said. For example, according to Hogwarts nurse Poppy Pomfrey, the unnamed curse that critically injured Hermione Granger during the Battle of the Department of Mysteries might have been fatal had Antonin Dolohov not been silenced when he used it.[3]

"Yes, those who progress in using magic without shouting incantations gain an element of surprise in their spell-casting. Not all wizards can do this, of course; it is a question of concentration and mind power which some ...lack."
-- Severus Snape in DADA Class, 6th Yea