Non-structured programming is historically earliest programming paradigm. A non structured program usually consists of sequentially ordered statements, usually one in each line. The lines are usually numbered or labeled to allow the flow of execution to jump to any line in program. There is no concept of procedures in non structured program; hence there are no independent reusable units in this programming paradigm. The program flow in non-structured programming would be as follows:

Structured design paradigm introduced the concept of selection and repetition of statements in code execution along with the line by line execution. It allowed writing of procedures and functions. These are the terms used for a block of code
that is written to perform a single task. Procedures and functions were the beginning of compartmentalization and hence re-usability of program code.
Procedures and functions which were for similar purpose were grouped together to get a module. A big software application consisted of multiple modules, each performing a particular task.

Structural design allowed modules to be reused in the form of code libraries.