The book describes that there are two conditions for equilibrium of rigid body, $$\sum \vec{F} = 0, \quad \sum \vec{M}_{o} = 0 $$ where $\sum \vec{F} $ is the vector sum of all the external forces acting on the body and $\sum \vec{M}_{o} $ is the sum of couple moments and moments of all the forces about any point O.
If we want to express these external forces and couple moments in Cartesian coordinate system , we get 6 equilibrium equations, $$ \sum F_{x} = \sum F_{y} = \sum F_{z} = 0$$ $$ \sum M_{x} = \sum M_{y} = \sum M_{z} = 0$$ I understand this, but in 2 dimensions, the equilibrium equations are $$ \sum F_{x} = \sum F_{y} = \sum M_{o} = 0.$$ Why are there only three equations? Why aren't there four, i.e. $$ \sum F_{x} = \sum F_{y} = \sum M_{x} = \sum M_y = 0.$$