We can divide the population of a nation into one of four categories.

A represents all those people aged between fifteen years old and under 65 years old, who are currently employed and working at least one hour per week.

B represents all those people aged between fifteen years old and under 65 years old, who are currently unemployed, but are looking for some form of employment.

C represents all those people aged between fifteen years old and under 65 years old, who are not employed and who are not looking for some form of employment.

D represents all those people aged under fifteen years and over 65 years, and other people excluded from A, B, and C for other reasons.

Clearly, the population of the nation is A + B + C + D.

The labour force are those people over fifteen years and less than sixty five years, who are either employed, or actively looking for employment.

That is, the labour force is defined as A + B