*Fixed costs. Costs that do not vary at all with volume, such as rent expenses.
*Variable costs. Expenses that vary directly and proportionately with changes in volume, for example, sales commissions.
* Semi-variable costs. Expenses that include both variable costs and fixed costs. These costs vary in the direction of, but not proportionately with, changes in the volume of sales, such a certain types of payroll as a firm becomes larger but do not change proportionally with sales changes.