The preceding rules ensure that the data type of an literal constant is completely determined by its form. Similarly the data type of an expression depends on the operands and operators involved. The intrinsic functions are also a special case, since their properties, including their data types, are known to the compiler. All other typed objects in a Fortran program are referred to by symbolic names. The rules given here apply to all of these named objects: variables, arrays, named constants, statement functions, and external functions.
In many programming languages, especially those in the Algol family, the data type of almost every item in the program has to be specified explicitly. Many programmers regard it as a chore to have to provide all these type specifications, although their presence does make it rather easier for the compiler to detect mistakes.
In Fortran you can specify data types explicitly in a similar way by using type statements, but Fortran also makes life easier by having certain default types. The data type of any object which has not been declared in a type statement depends on the first letter of its name. The default rules are:
| First letter of the name | Implicit type |
| A to H | REAL |
| I to N | INTEGER |
| O to Z | REAL |
Most programs make extensive use of integer and real objects, so these default values reduce the number of type statements that are required, provided suitable initial letters are chosen for the symbolic names.
The first-letter rule can also be changed throughout a program unit by using an IMPLICIT statement, described below.