A floating point type supporting about 15 digits of precision System unit
  type Double;
The Double type is the general purpose floating point type in Delphi. It strikes the balance between capacity/precision versus storage/performance.
It supports approximately 15 digits of precision in a range from 2.23 x 10-308 to 1.79 x 10308.
The Single type is smaller and faster, but with reduced capacity and precision.

The Extended type has the highest capacity and precision, but biggest storage and worst performance.

A Double set to its highest value is treated as Infinity.
Related commands
Currency A decimal type with 4 decimal points used for financial values
Extended The floating point type with the highest capacity and precision
Single The smallest capacity and precision floating point type
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Example code : Showing the precision and capacity of Double values
  account1, account2, account3, account4 : Double;
  account1 := 0.1234567890123456789;      // 20 decimal places
  account2 := 1.234567890123456789E308;   // Highest exponent value
  account3 := account1 + account2;
  account4 := 9.9E308;                    // Treated as infinite

  ShowMessage('Account1 = '+FloatToStr(account1));
  ShowMessage('Account2 = '+FloatToStr(account2));
  ShowMessage('Account3 = '+FloatToStr(account3));
  ShowMessage('Account4 = '+FloatToStr(account4));

Show full unit code
   Account1 = 0.123456789012346
   Account2 = 1.23456789012346E308
   Account3 = 1.23456789012346E308
   Account4 = INF
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