Said of pairs of apparent opposites such as Form and Emptiness， Birth and Death， Noumenon and Phenomena which are concurrent aspects of the same non-dual reality. Thus these opposites always go together -- they are said to be born simultaneously， to have dual birth. The most famous example is found in the Heart Sutra: Form is emptiness， Emptiness is form.
Illustrative Story
"Once a beautiful and well-dressed woman visited a house. The master of the house asked her who she was; and she replied that she was the goddess of wealth. The master of the house was delighted and so greeted her with open arms. Soon after another woman appeared who was ugly looking and poorly dressed. The master asked who she was and the woman replied that she was the goddess of poverty. The master was frightened and tried to drive her out 'of the house， but the woman refused to depart， saying， 'The goddess of wealth is my sister. There is an agreement between us that we are never to live apart; if you chase me out， she is to go with me.' Sure enough， as soon as the ugly woman went out， the other woman disappeared. Birth goes with death. Fortune goes with misfortune. Bad things follow good things. Men should realize this. Foolish people dread misfortune and strive after good fortune， but those who seek Enlightenment must transcend both of them and be free of worldly attachments." （The Teaching of the Buddha）