Z_2\times Z_2$ orbifold compactification as the origin of realistic free fermionic models

I discuss the correspondence between realistic four dimensional free fermionic models and $Z_2\times Z_2$ orbifold compactification. %I illustrate this correspondence for one particular example. I discuss the properties of the $Z_2\times Z_2$ orbifold that are reflected in the realistic free fermionic models. I argue that the properties of the realistic free fermionic models arise due to the underlying $Z_2\times Z_2$ orbifold compactification with nontrivial background fields. I suggest that three generation is a natural outcome of $Z_2\times Z_2$ orbifold compactification with ``standard embedding'' and at the point in compactification space that corresponds to the free fermionic formulation. I discuss how quark flavor mixing is related to the compactification.