  The realistic free fermionic models have had remarkable success in providing
plausible explanations for various properties of the Standard Model which
include the natural appearance of three generations, the explanation of the
heavy top quark mass and the qualitative structure of the fermion mass spectrum
in general, the stability of the proton and more. These intriguing achievements
makes evident the need to understand the general space of these models. While
the number of possibilities is large, general patterns can be extracted. In
this paper I present a detailed discussion on the construction of the realistic
free fermionic models with the aim of providing some insight into the basic
structures and building blocks that enter the construction. The role of free
phases in the determination of the phenomenology of the models is discussed in
detail. I discuss the connection between the free phases and mirror symmetry in
(2,2) models and the corresponding symmetries in the case of the (2,0) models.
The importance of the free phases in determining the effective low energy
phenomenology is illustrated in several examples. The classification of the
models in terms of boundary condition selection rules, real world-sheet fermion
pairings, exotic matter states and the hidden sector is discussed.
