Tatyar was the original name of the second Clan of the Elves, before the Great March. The Clan-name, meaning 'Seconds' (q. cardinal (t)atya 'second') was derived from the name of the father of the Clan, Tata 'Two'. Tata was the second Elf-father tho awake at Cuiviénen.
In Cuiviénen they were the 56/144 of the total population. Half of them followed Finwë and became the Noldor and half of them stayed back. The other original clans were Minyar 'Firsts', and Nelyar 'Thirds' according to the Elf-fathers Imin 'One' and Enel 'Three'.