Definition
If the transformation function F is differentiable, and if F(y) = 0,
(Note: y is a production vector), then the Marginal rate of transformation (MRT) between good l and good k at 
:
Holding 
fixed.
Note: 
are goods in production vector, they may and may not be inputs.
Interpretation
How much the (net) output of good k can increase if the firm decreases the (net) output of good l by one unit.
(Image the graph where l is on horizontal axis, k is on vertical axis.)
Slope of tangent line of transformation frontier is 
(Good 1 on horizontal axis, good 2 on vertical axis)
(slope is 
)
The MRT to two inputs is the marginal rate of technical substitution (MRTS)