Thomas Sutcliffe: What's in a name? Quite a lot

I heard Rachel Whiteread the other day on Front Row explaining why she'd called her new installation in the Tate Turbine Hall Embankment. Artists sometimes get a bit cagey about dissecting their own work, but - despite a hard day with the kind of media outlets that contemporary art doesn't usually reach - she was straightforward and obliging.

Embankment was partly a nod to the Tate's London address, she explained, but she also liked the fact that it referred to a physical process; the banking up of material into a wall or slope. It's certainly a bit more expressive than some of her titles in the past, which - with the exception of the occasional foray into the transcendental (one of her room interior casts was called Ghost) - have tended to be bluntly utilitarian.

Like many contemporary artists, she is fond of Untitled, though the consumer-unfriendliness of that word is often softened by an additional description: Untitled: Bed, say, or Untitled: 24 Switches. Perhaps she's realised that big public pieces of art need something a little richer, though. The formal title for her piece for the fourth plinth in Trafalgar Square was Untitled Monument - but it soon became "Plinth" in the press, an irresistible provocation to cheesy puns. In the popular understanding, things must have names. If they don't, they tend to get nicknames instead.

In other words, Embankment - non-committal, mildly ambiguous, neutral - effectively occupies a space that might otherwise have been filled with something much less desirable. Whiteread tagged her creation in order that less well-disposed graffiti artists wouldn't vandalise it with their own inventions. And who can blame her, when you consider how difficult it can be to remove the traces of a hastily scribbled phrase?

Upriver at Tate Britain, there's a classic example of the damage that can be done in the last room of Degas, Sickert and Toulouse-Lautrec, a show that aims to overturn the notion of British aesthetic insularity at the turn of the century. The exhibition ends with a matched pair of paintings - one by Sickert, one by Degas, painted nearly 50 years apart but linked by their similar subject-matter. Both show artificially-lit rooms containing two figures, a man half turned towards us and a woman half turned away. And with Sickert's painting there's no doubt about the mood he's after, because he's called his picture Ennui, a title that not only gives the painting a helpful coat of continental varnish but makes it virtually impossible to look at it with any ambivalence. It's about being stifled - and that glass on the table in the foreground is half empty, not half full.

I'm willing to bet, though, that if this picture were called Reverie or Contentment, viewers would have no difficulty in reading an expression of contemplative pleasure into the face of the man enjoying his cigar. So Sickert's title isn't just an accessory to the piece, it's an essential part of it.

Degas' title is rather different - Interior (The Rape). The painter gave it the first half, and the second half somehow became attached to it as a nickname. Interior is a kind of 19th-century figurative equivalent of Untitled, and it is exactly right for the studied ambiguity of Degas' image, which is one of his most un-Degas-like pictures.

Clearly something has just happened here, and we're looking at an aftermath. But what's so great about the picture is that it is impossible to say categorically whether an offence has been committed, or even who has committed it. Our gender prejudices (all men are brutes) naturally incline us to see him as the perpetrator, blocking the woman's only exit from the room. But isn't there something scornful about her posture, and something doggily chastened about his? Could it not be that she's fierce and he's hurt?

And don't even start on that box on the table, hooking the eye with its bright red interior but offering nothing in the way of substantive evidence. If it contains proof, whose guilt does it point to? I haven't a clue, really, which is surely what Degas wanted. This is a painting about the way reality forks after a row; there are never fewer than two accounts of what happened and they can hardly ever be reconciled. It's also about how you can't be sure - from the outside - what an interior contains.

And then someone comes along and calls it The Rape, destroying that shimmer of uncertainty with one crude stroke. This title isn't a negligible accessory to the picture, either. It's a screen through which you can only dimly glimpse the subtleties of the original, and, unfortunately, once you've seen that word, it's a screen that takes a real effort to remove. Couldn't the Tate help by taking it off the label - or has this bit of graffiti become irremovable?