In 1960, the metre was redefined in terms of the wavelength of a particular spectral line of krypton-86, and, in 1967, the second was redefined in terms of the hyperfine transition frequency of the ground state of caesium-133.
1960'ta metre krypton-86 spectral düzleminin belli bir dalgaboyuna göre yeniden tanımlandı ve 1967'de saniye caesium-133'ün ince geçiş frekansına göre yeniden tanımlandı.