perfect substitutes - linear isoquants  

• isoquants
• constant MRTS
• diminishing MRTS doesn't apply
• not necessarily a one to one exchange though

fixed-proportions production function - like perfect complements in consumer theory  

• isoquants
• impossible to make substitutions among inputs (ie. recipes)
• each output requires a specific combo of inputs
• both inputs must be increased to increase output >> limited methods of production

returns to scale - shows how output is increased by input  

• increasing returns to scale - output more than doubles when inputs doubled
  • for example, Q = KL >> (2K)(2L) = 4KL = 4Q
  • common in large scale operations (w/ very specialized operations)
• constant returns to scale - output doubled when inputs doubled
  • for example, Q = K+L >> (2K)+(2L) = 2(K+L) = 2Q
  • size of firm doesn't affect productivity
• decreasing returns to scale - output less than doubled when inputs doubled
  • for example, Q = (KL)^1/3 >> (2K x 2L)^1/3 = 4^1/3Q