The slope of any isoquant is the **MRTS** which turns out to the the ratio of the **Marginal Product of Labor** to the **Marginal Product of Capital**. Remember that the **MP**_{L} is the added output due to a small increase in labor while the **MP**_{K} is the added output due to a small increase in capital. The ratio of these two will tell us at what rate capital and labor may be substituted for one another while maintaining output.
Suppose ** - MP**_{L}/MP_{K} = 2. This would mean that if labor were reduced by one worker capital would need to be increased by 2 units to maintain output (remain on the isoquant). Since the isoquant is a curve the **MRTS** is different at each point.

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