Karnaugh Maps in POS instead of SOP
Given PoS:  \$ f=\overline{C} \space \overline{D} +A\overline{D} + A\overline{B}\$

Becomes SoP:  \$ f= (\overline B + \overline{D})\cdot (A + \overline C)\cdot (A + \overline D ) \$ 

Circle 0's. Write OR'ed equations with inverted inputs.  

Intersection of B &amp; D have 4 zeros.  This becomes \$ \overline B + \overline D\$