Between 1960 and 1983 the metre was defined as being equal to 1 650 763.73 wavelengths of an orange emission line of krypton-86. How did they actually use that definition? Was it possible to move the mirror of an interferometer slowly over a distance of 1 metre, perhaps by a very smooth screw, and count the appearance of 1.6 million fringes without mistakes? Was the coherence length of the krypton light more than 1 metre?