The ridge lines in a production function diagram define:

Ridge lines separate the technically efficient or economic region of production from technically inefficient regions. Upper Ridge Line: MPK = 0 At this boundary, the marginal product of capital becomes zero. Beyond the ridge line, MPK becomes negative. Lower Ridge Line: MPL = 0 At this boundary, the marginal product of labour becomes zero. Beyond the ridge line, MPL becomes negative. Inside the ridge lines, both marginal products are positive: MPK > 0 MPL > 0 Therefore, the region between the ridge lines represents the economically meaningful region of production.