Special techniques are thus required to measure the positions of the surface atoms, and these generally fall into two categories: diffraction-based methods adapted for surface science, such as low-energy electron diffraction (LEED) or Rutherford backscattering spectroscopy, and atomic-scale probe techniques such as scanning tunneling microscopy (STM) or atomic force microscopy. Of these, STM has been most commonly used in recent history due to its very high resolution and ability to resolve aperiodic features.