To give provisional names to his predicted elements, Mendeleev used the prefixes eka-,[note 1] dvi-, and tri-, from the Sanskrit names of digits 1, 2, and 3, depending upon whether the predicted element was one, two, or three places down from the known element of the same group in his table. For example, germanium was called eka-silicon until its discovery in 1886, and rhenium was called dvi-manganese before its discovery in 1926.
The eka- prefix was used by other theorists, and not only in Mendeleev's own predictions. Before the discovery, francium was referred to as eka-caesium, and astatine as eka-iodine. Sometimes, eka- is still used to refer to some of the transuranic elements, for example, eka-actinium (or dvi-lanthanum) for unbiunium. But current official IUPAC practice is to use a systematic element name based on the atomic number of the element as the provisional name, instead of being based on its position in the periodic table as these prefixes require.