In the Harry Potter books, what do wizards wear under their robes?

In the movies, they wore ties and really nice shirts, but I don't think that's how it was in the books?

What do wizards and witches wear under their robes?

Answer

In the books, there's basically no mention of wizards wearing anything under their robes other than underpants (or in extreme cases nothing at all).

Order of the Phoenix

Snape was hanging upside-down in the air, his robes falling over his head to reveal skinny, pallid legs and a pair of greying underpants.

Prisoner of Azkaban

Harry took off his black school robes, removed his wand from his pocket, and stuck it inside the T-shirt he was going to wear under his Quidditch robes

Deathly Hallows (edited by me for brevity)

She led them down a side street, then into the shelter of a shadowy alleyway. ‘When you say you’ve got the Cloak, and clothes …’ said Harry, frowning at Hermione, who was carrying nothing except her small beaded handbag, in which she was now rummaging. ‘Yes, they’re here,’ said Hermione, and to Harry and Ron’s utter astonishment, she pulled out a pair of jeans, a sweatshirt, some maroon socks and, finally, the silvery Invisibility Cloak.

Ron, hurry up and change …’....‘When did you do all this?’ Harry asked, as Ron stripped off his robes....‘You’re amazing, you are,’ said Ron, handing her his bundled-up robes....

[note the lack of mention of him covering his little wizard while he changed]

Goblet of Fire

Harry, Ron and Hermione joined it, right behind a pair of men who were having a heated argument. One of them was a very old wizard who was wearing a long flowery nightgown. The other was clearly a Ministry wizard; he was holding out a pair of pinstriped trousers and almost crying with exasperation. ‘Just put them on, Archie, there’s a good chap, you can’t walk around like that, the Muggle on the gate’s already getting suspicious –’ ‘I bought this in a Muggle shop,’ said the old wizard stubbornly. ‘Muggles wear them.’ ‘Muggle women wear them, Archie, not the men, they wear these,’ said the Ministry wizard, and he brandished the pinstriped trousers. ‘I’m not putting them on,’ said old Archie in indignation. ‘I like a healthy breeze round my privates, thanks.’

and

All their teammates but Angelina were already in the changing room when they entered. [...] ‘Shut up,’ said Ron, stony-faced, pulling on his own team robes for the first time. They fitted him well considering they had been Oliver Wood’s, who was rather broader in the shoulder.

[Again, note that he didn't feel the need to cover up despite Alicia Spinnet being in the room]

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UPDATE : It has been pointed out to me that in the French version of the novels, Ron is wearing a "pantalon" (e.g. pleated slacks) under his robe:

ses chaussures et le bas de son pantalon - _{^{Harry Potter a L'ecole Des Sorciers}}