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Read past the £12bn number, and the 46 per cent discount, and you’ll come to some figures that are just as shocking.

They are, as has become customary, tucked into the credit market exposures section. The top line is that RBS is estimating that it has suffered £4.3bn net writedowns in 2008, or £5.9bn before tax.

But it’s the detail that fascinates. Back in February, when RBS reported its full-year results, the bank’s portfolio of Alt-A assets barely merited a mention. It was 85 per cent investment grade after all. Now that £2.2bn book has had its value slashed by half.

Elsewhere the marks are brutal, especially given RBS’ rather rosy spin at the results two months ago. The CDO portfolio has been annihilated. Subprime RMBS, three-quarters of which was deemed “investment grade” back in December, is now reduced to 38 per cent on average.

RBS’s remarkably resilient leveraged loan book, which took just a five per cent hit at the end of 2007, has capitulated with a mark down to 88 per cent. Doubly odd considering the tentative improvement in the market in recent weeks, judging by derivatives indices and the dwindling backlog of loans on banks’ books.

It’s the kind of emergency kitchen-sinking that makes these RBS figures seem like distant cousins of those issued back in February. They’re related. But only just.

Reading across to other banks can be hazardous, say analysts at Collins Stewart. “Risk is not homogenous” as BarCap’s Bob Diamond would have it. Sir Fred Goodwin said something very similar back in February, we seem to remember.

But taking similar valuation levels would amount to a £6bn to £7bn hit at Barclays, they estimate, plus another £2bn of capital required to take Barclays to the new gold standard core Tier 1 of 6 per cent.

HBOS has a £7.1bn Alt-A book - and took writedowns of just £227m last year. This is the stuff, US-asset backed, that is not eligible for free parking at the Bank of England.

The bank’s exposure to ‘credit risk assets’ is about the same as Lloyds TSB’s relative to its adjusted assets, say Cazenove. As a multiple of its equity Tier 1 though, it is higher. But in terms of maintaining the bank’s capital position, adds Cazenove, only £2.4bn of its Alt-A is held in the trading book. The remainder, in its banking book, will be marked-to-market through the ‘available for sale’ reserve. It only hits regulatory capital if the loss is realised, for example through a sale or because of underlying defaults.

In terms of actual underlying losses, rather than market value losses, they add:

The Alt-A RMBS notes that HBOS owns are all AAA-rated; the devaluation of this rating should not detract from the fact that it means HBOS’ notes benefit from subordination within the RMBS structure. Management states that this is 30%. This means realised losses on the underlying portfolio must exceed this level before HBOS’ notes incur losses. If this level were breached, however, then the geared nature of a RMBS means the potential loss to HBOS would rise sharply.

The bottom line is that Cazenove believe that realised losses on Hbos’ Alt-A are unlikely. But every 10 per cent fall in market value is good for a 5bp knock to Tier 1, via the £2.4bn trading book. That was enough to put Hbos at the top of the fallers list in the FTSE100.

Related links
RBS statement
The RBS conference call - FT Alphaville
A Way Charges Stay Off Bottom Line - WSJ