In a molecule, an applied magnetic field, B0, causes the electron density surrounding the nuclei to circulate, setting up a local diamagnetic current, which induces a local magnetic field, Blocal, opposing B0. So, the effective magnetic field experienced by these nuclei, Beffective, equals B0 minus Blocal, in a phenomenon called local diamagnetic shielding. Essentially, Blocal increases with the electron density surrounding the nuclei, leading to increased shielding and lower Beffective. Since electron densities vary within a molecule, each nucleus is shielded to a different extent and experiences a different effective field. A nucleus in an electron-dense environment is well-shielded from the applied magnetic field and experiences a lower Beffective. So, the energy required to flip its spin is less than that required for a poorly-shielded nucleus in electron-poor surroundings. As a result, shielded nuclei have lower absorption frequencies than deshielded nuclei.