David Arthur is correct in saying that the number of noughts in big numbers is important (Letters, November 27). However, a billion is described in the Oxford English Dictionary as being a million million. Forty years ago, Prime Minister Harold Wilson decided we should adopt United States usage and define a billion as a thousand million. He did this for the sake of simplicity in dealing with large numbers internationally. However, it is evident that many politicians and political commentators still show a lamentable lack of grasp of what is really very basic arithmetic. It might help, perhaps, if they considered the following: one million seconds is a bit over 11 days; one billion seconds is a bit over 32 years; one trillion seconds is more than 32,000 years.

David Arthur is correct in saying that the number of noughts in big numbers is important (Letters, November 27). However, a billion is described in the Oxford English Dictionary as being a million million. Forty years ago, Prime Minister Harold Wilson decided we should adopt United States usage and define a billion as a thousand million. He did this for the sake of simplicity in dealing with large numbers internationally. However, it is evident that many politicians and political commentators still show a lamentable lack of grasp of what is really very basic arithmetic. It might help, perhaps, if they considered the following: one million seconds is a bit over 11 days; one billion seconds is a bit over 32 years; one trillion seconds is more than 32,000 years.

Hugh Dunnachie, Wellstrand Cottage, Sanquhar.

I am grateful to David Arthur for drawing attention to the confusing issue of the definition of a trillion by citing my use of it in my letter (November 26). In my first letter I referred to Samir Amin's original article, which for understandable reasons of space was not included in the published version.

Professor Amin makes it clear that he is using the (American) English definition of a trillion as a thousand millions. This are, indeed, "three naughts lost" from a million million. He has clearly defined his terms, which I then used in my letter.

The use of "trillions" is internally consistent. But Mr Arthur is correct. Numbers do matter.

Dr John O'Dowd, Downfield Gardens, Bothwell.

I, too, noticed John O'Dowd's erratum in the number of zeros in his trillion but, unlike David Arthur, it did not cross my mind that such a cogent writer lacked a grasp of magnitude. Those of us in science who constantly deal with large numbers tend to avoid the repetition of zeros where it is so easy to slip.

Several alternative methods of writing the digits of a number are in use. For example, 6000 may be written 6 x 10E3, six times 10 with an exponent three. It is further commonly abbreviated to 6E3, the base 10 being understood.

The International System of Units, the modern form of the metric system, does excellent service by naming large and small numbers in an internationally-defined way. Thus, one million pounds could be a mega pound, one billion pounds a giga pound and one trillion pounds a tera pound. If the credit crunch worsens we may need even larger numbers. One thousand trillion pounds would be a peta pound.

Chris Parton, Bellshill Road, Uddingston.