In his 1949 doctoral thesis, Abelès developed a transfer-matrix formalism to compute the transmission and reflection of light by thin dielectric layers. When Lyman G. Parratt performed the first x-ray reflectometry experiment in 1954, he developed an equivalent recursion method. Nowadays, Abelès matrices or Parratt recursion are exchangeably used in investigations of multilayers by x-ray reflectometry, neutron reflectometry, or ellipsometry.