The specification of production time periods is a convenient way to understand and explain production activity by a firm, which then provides insight into market supply. The standard distinction is generally between short run, with at least one fixed and one variable input, and the long run, with all inputs variable. However, in some cases, the very short run or market period, with all inputs fixed, is the proper time period. And in other circumstances it is useful to consider the very long run, with inputs beyond the control of the firm also variable.

(i) Short Run:

The short run is the production time period in which at least one input under the control of the firm is variable and at least one input is fixed. This time period is relevant for short-run production analysis. In particular, with one fixed and one variable input, the law of diminishing marginal returns guides short-run production "and determines how a firm responds to changes in the market price.

(ii) Very Short Run:

The very short run, or market period, is the production time period in which all inputs under the control of the firm are fixed. With all inputs fixed, the quantity of output produced is also fixed. In other words, the firm has produced the output and now is concerned only with selling. It cannot produce more or less, it can only sell what it has.

(iii) Long Run:

The long run is the production time period in which all inputs under the control of the firm are variable. This time period is relevant for long-run production analysis. In particular, with all inputs variable, long-run production is guided by returns to scale rather than marginal returns and the law of diminishing marginal returns. Thus, it is a period which can allow a firm to make concrete adjustment on its plans or output s to meet market expectations without necessarily changing the technology.

(iv) Very Long Run:

The very long run is the production time period in which all inputs are variable, including those under control of the firm and those beyond the control of the firm. During this time period, key production inputs such as government rules, technology, and social customs also change. In most cases, the analysis of the very long run is concerned with how changes in technology affect a firm's production.