Both scattering data and spectroscopic data are used to determine the CODATA recommended values for the proton and deuteron root-mean-square charge radii. Furthermore, spectroscopic measurements can be made both with regular hydrogen (consisting of a proton and an electron) or muonic hydrogen (an exotic atom consisting of a proton and a negative muon). An inconsistency between proton charge radius measurements made using different techniques was known as the proton radius puzzle, but more recent measurements show consistent results.