Why are Annexin V conjugates and 7-AAD used together?

Using Annexin V conjugates and 7-AAD (7-amino-actinomycin D) together is the most effective way to differentiate early apoptosis from late apoptosis and necrosis.  

Annexin V conjugates are an efficient label for staining phosphatidylserines (PS), which are expressed on the outer layer of the plasma membrane by early apoptotic cells. 

On the other hand, 7-AAD is a membrane-impermeant fluorescent dye that is unable to reach the DNA in viable cells. It is only capable of binding to DNA in late apoptotic cells and necrotic cells that have lost their membrane integrity. 

Hence, using both labels together allows scientists to distinguish early apoptosis from late apoptosis and necrosis.