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Sample Custom Number Formats
Format Code Description Data with General Format Data with Custom Number Format
       
[>9999999](000)000-0000;000-0000 Displays a telephone number with or without an 7045556325 (704)555-6325
  area code. 9106325689 (910)632-5689
  If the number is greater than 9,999,999, this code 8896523 889-6523
  displays the number with an area code 5362563 536-2563
  ((000)000-0000); otherwise the number appears 2065896325 (206)589-6325
  without the area code (000-0000). 3369856 336-9856
       
[<1].00¢;$0.00_¢ Shows currency values in dollars or cents. 1.25 $1.25
  This code displays values less than 1 in 3 $3.00
  cents notation (.00¢), and display values 0.35 .35¢
  greater than or equal to 1 in dollars, and 0.95 .95¢
  leaves a space on the right so that the 22.36 $22.36
  decimals line up ($0.00_¢). 0.75 .75¢
       
[<=2]"Low"* 0;[>=4]"High"* 0;"Average"* 0 Using "If, ElseIf, Else" in a number format: 1 Low                1
          If the value is <=2, display the word "low" 2 Low                2
                  with the value, 3 Average          3
          Else If the value is >=4, display the word 4 High               4
                  "high" with the value, 5 High               5
          Else display the word "Average" with    
                  value.    
       
[Red][<=2]0;[Green][>=4]0;[Black]0 Using "If, ElseIf, Else" in a number format: 1 1
          If the value is <=2, display the value 2 2
                  with red text, 3 3
          Else If the value is >=4, display the value 4 4
                  with green text, 5 5
          Else display the value with black text.    

This table was produced from the example Excel file available at http://support.microsoft.com/support/Excel/Content/Formats/default.asp