The pressure in a liquid at two points in the same horizontal plane are equal. 

Consider an elevator accelerating upward and a car accelerating on a horizontal road. The above statement is correct in

• When two places in the same horizontal plane accelerate upward, as in an elevator, the pressure at those two spots is the same since both of those sites experience the identical pseudoforces.

              ${\mathrm{P}}_1={\mathrm{P}}_2={\mathrm{P}}_0+{\mathrm{\rho g}}_{\mathrm{eff}}\mathrm{h} \Rightarrow ∆\mathrm{P}=0$

• However, as a car accelerates horizontally, inertia will cause the liquid above the points to shift height, which will change pressure at two horizontal points.

             $∆P=\rho La\ne 0$   ( where, L is the distance between the two points and a is the acceleration of the fluid)