The COBOL makes use of the level number concept to explain the hierarchical data structure.
A level number is a 2 digit number. The permitted level numbers are 01 - 49, 66, 77 and 88.
All the variables are declared in the working-storage section by using appropriate level numbers.
The elementary data item can acquire either 01 or 77 as its level number. In the COBOL a division is made between the elementary and group data items. A few elementary data may be joint to form a group.
For illustration, the DAY, MONTH & YEAR may be 3 elementary data items. These may be joint to form a group data named DATE. The association may be shown pictorially as shown below:
It may be noted that the memory space referred to by the DATE is the combined memory space for DAY, MONTH &YEAR. The benefits of such a grouping are very clear. The programmer can now refer to the individual elementary items DAY, MONTH, YEAR or the group item DATE. An elementary data item is however the one which the programmer would always like to refer to as an entire and not in parts.