4d and 5d Series
The d-block takes place the large middle section flanked through s- and p- blocks in the periodic table. The name 'transition' provided to the elements of d-block is just due to their position among s- and p- block elements.
The penultimate energy level's d-orbitals in their atoms accept electrons giving rise to the three rows of the transition metals, that is, 3d, 4d and 5d. The fourth row of 6d is still not complete.