This excerpt was taken from Sun Tzu at Gettysburg by Bevin Alexander. Click here to purchase it from Amazon.com.
Sun Tzu's Zheng and Qi
Excerpt from Sun Tzu at Gettysburg, by Bevin Alexander, page 127
Practically all successful commanders throughout history have applied [Sun Tzu’s] rule of zheng [orthodox forces] and qi [unorthodox forces]. Combining these two approaches causes the enemy to divide his own strength, allowing the decisive blow to be delivered against a fraction of the enemy’s power, not---as in the case of a direct attack---against the vast bulk of his power.
“In warfare,” Sun Tzu writes, “the strategic configurations of power do not exceed the unorthodox and the orthodox, but the changes of the unorthodox and the orthodox can never be completely exhausted. The unorthodox and orthodox mutually produce each other, just like an endless cycle. Who can exhaust them?”
In general, the orthodox is the main force that engages the enemy and holds him in place, while the unorthodox is the smaller force that attacks the enemy at a different, unexpected, place, usually flank or rear, forcing him to disintegrate.