A number format can have up to three different definitions, for positive and negative numbers and for zero. If you do not define a separate way of formatting negative numbers, they will be displayed in the same format as positive numbers but with a minus sign in front. In practice you need only specify a second format if you want negative numbers to be formatted differently from positive numbers. For example, in a financial worksheets number format it may be useful if negative numbers appear enclosed by brackets.
To create separate number formats, enter each format in the number format box separated by a semi-colon. The negative format appears after the semi-colon. So a format for currency with negatives in brackets would be:
£#,##0.00;(£#,##0.00)
Enter a second semi-colon after the first two formats and a third format can be entered to format the number 0. If you enter a second semi-colon and no format, 0 will not be displayed. Equally, if you enter a semi-colon after the first number format, but no special format for negative numbers, negative numbers will not be displayed on the worksheet.