When the basis set has two/three sets of basis functions to represent each orbital, they are called double/triple-zeta (DZ)/TZ) basis set. For example, two s-functions (1s and 1s') are used as DZ basis set for the hydrogen atom. The first row of the element in the periodic table needs four sets of s-functions (1s, is', 2s, 2s') and two sets of p-functions (2p, 2p') for DZ basis sets.