Draw the angle in standard position: it has its initial side on the x-axis and moves counter-clockwise to the terminal side. The terminal side is in the third quadrant, because the angle measure is between $\displaystyle{{180}}^{\circ}\usepackage{cmbright}\Huge\$ (the negative x-axis) and $\displaystyle{{270}}^{\circ}\usepackage{cmbright}\Huge\$ (the negative y-axis). In particular:

$\displaystyle{{240}}^{\circ}={{180}}^{\circ}+{{60}}^{\circ}\usepackage{cmbright}\Huge\$

This tells you that the terminal side is $\displaystyle{{60}}^{\circ}\usepackage{cmbright}\Huge\$ below the negative x-axis. To say this another way, the x-axis and the terminal side form a $\displaystyle{{60}}^{\circ}\usepackage{cmbright}\Huge\$ angle. This is the reference angle.