I was never quite sure what trillion were. They used to turn up in Italy in the days of the lira.

When you asked Italians how many noughts one contained, those that attempted an answer at all said they thought it was more than a million, which didn't take things much further.

Now we know. The Bank of England published numbers yesterday showing that British home-buyers owe more than £1,000 billion in mortgages.

People in the know then announced that home-buyers now owed more than a trillion. In other words, it is a million million - what we used to call a billion before we caved in to American usage and trimmed three noughts off the end to make it a thousand million.

Whether a 13-digit mort-gage mountain actually matters to anyone except those who have taken on more than their prudent share of it - and banks that have competed too fiercely to lend to them - is something else.

One who fears it does is the respected Liberal Democrat Shadow Chancellor, Vince Cable. He has been sounding off for more than two years about reckless lending and the economic as well as social consequences of spiralling personal debt.

Yesterday he claimed there are two million British households living on a financial knife-edge, plus half a million already with serious financial problems. Any interest rate increase could be disastrous.

The Bank of England disputes this. It sticks with the view that excessive unsecured credit - not the trillion pounds of mortgages - could become a social problem with much personal distress, but that there is not enough of it to mess up the economy.

Still, the actual number for outstanding unsecured debt has now reached £192 billion, nobody's trivial sum. Mr Cable works it out as £3,250 per person.

It represents a culture in which credit is a normal feature of life. Small wonder when the Government has ordained that half the young population shall start adult life with a bundle of student debt.

The FSA's chief executive John Tiner, has spoken solemnly about the ability to manage money growing with age and experience.

It does. But confident young borrowers are convinced they know what they are doing, that their parents (well, grandparents) lived in a different world.

Many see no point in saving up a deposit for a home of their own because they don't see how they will ever afford it. As to pensions, apart from being over the horizon, if you believe the headlines, they don't work.

The banks have devised credit scoring systems that encourage them to lend more freely than ever before - so far successfully. It is entirely possible that both they and most of their customers are right.

The weak spot may turn out not to be unsecured lend-ing at all, but the explosion of high-priced "sub-prime" mortgages, £30 billion of them last year alone, all charging premium rates to people more likely than most to get into a financial fix.

The hazard is that if too many of these get in to trouble at once they could tip over the housing market.