Nazgûl clothes: visible and invisible

[Lalwendë]

Immediately, though everything else remained as before, dim and dark, the shapes became terribly clear. He was able to see beneath their black wrappings. There were five tall figures: two standing on the lip of the dell, three advancing. In their white faces burned keen and merciless eyes; under their mantles were long grey robes; upon their grey hairs were helms of silver; in their haggard hands were swords of steel. Their eyes fell on him and pierced him, as they rushed towards him. Desperate, he drew his own sword, and it seemed to him that it flickered red, as if it was a firebrand. Two of the figures halted. The third was taller than the others: his hair was long and gleaming and on his helm was a crown. In one hand he held a long sword, and in the other a knife; both the knife and the hand that held it glowed with a pale light. He sprang forward and bore down on Frodo.

I always think that the Ringwraiths are a constructed, twisted, mortal equivalent of the 'houseless fea'. Sauron's Rings clearly have an effect upon the hroa and fea of the bearer, and I think the rings he made for Men give a false impression of immortality by working on the nature of Men's being, even wearing their hroa to the point of invisibility or being as insubstantial as to barely exist. This was Sauron's way of making the seven bearers 'immortal' - their hroa are changed in nature so that they fade and do not die.

What I think Frodo is seeing is the 'real' Men behind the Ringwraiths and what they have become in their thousands of years of entrapment. Therefore the 'robes' and 'crowns' are literally shadows of the past which Frodo is able to see as he has entered that world by donning the Ring (the One Ring gives an instant access to this netherworld, whereas presumably you'd need to wear one of the seven rings for a while before you succumbed). What he 'sees' is not entirely real as note that he also sees his own sword flickering red, not blue.

I don't think it would actually matter to a Ringwraith whether he wore 'clothes' under his black robes anyway, as would the chafe of horseriding literally bareback cause any discomfort to a Wraith? Trousers or the lack of them probably wouldn't bother your everyday Wraith.