When the production possibility frontier is a straight line, the opportunity cost of a pound of cheese in terms of wine is constant. As we saw in the previous section, this opportunity cost is defined as the number of gallon of wine the economy would have to give up in order an extra pound of cheese. In this case, to produce another pound would require a_LC person – hours. Each of these person – hours could in turn have been used to produce (_a^1)LW gallons of wine. Thus the opportunity cost of cheese in terms of wine is a_LC/a_LW. For example, if it takes one person – hours to make a pound of cheese and two hours to produce a gallon of wine, the opportunity cost of cheese in terms of wine is one-half. As figure 2-1 shows, this opportunity cost is equal to the absolute value of the slope of the production possibility frontier.