  Two dimensional (2D) and three dimensional (3D) Rayleigh-B\'enard convection
is compared using results from direct numerical simulations and prior
experiments. The explored phase diagrams for both cases are reviewed. The
differences and similarities between 2D and 3D are studied using Nu(Ra) for Pr
= 4.38 and Pr = 0.7 and Nu(Pr) for Ra up to $10^8$. In the Nu(Ra) scaling at
higher Pr, 2D and 3D are very similar; differing only by a constant factor up
to $\text{Ra} = 10^{10}$. In contrast, the difference is large at lower Pr, due
to the strong roll state dependence of Nu in 2D. The behaviour of Nu(Pr) is
similar in 2D and 3D at large Pr. However, it differs significantly around Pr =
1. The Reynolds number values are consistently higher in 2D and additionally
converge at large Pr. Finally, the thermal boundary layer profiles are compared
in 2D and 3D.
