I will discuss the classical 4d scalar potential in type IIA Calabi-Yau orientifolds in the presence of fluxes and D6-branes. As we will see, such a potential can be written as a bilinear form V = Z_AB ρ^Aρ^B, where the ρ are in one-to one correspondence with the fluxes of the compactification. The ρ only depend on the internal fluxes, the axions and the topological data of the compactification, and are fully determined by the Freed-Witten anomalies of branes that appear as 4d string defects. The quadratic form Z only depends on the saxionic partners of these axions. In general, the ρ can be seen as the basic invariants under the discrete shift symmetries of the 4d effective theory, and therefore the building blocks of any flux-dependent quantity.