Portfolio theory tries to the explain the equilibrium rate of return or the price fixation in capital market through the two important relationship these include:

1) capital market line (CML)

2) security market line (SML)

While capital market line (CML) tries to exhibit the linear relationship between the risk and relationship risk and return in the case of portfolio the security market line (SML) provides an explanation on how individual securities are price based on the size of the systemic risk that each security possess.