The image to the right shows the first few hydrogen atom orbitals (energy eigenfunctions). These are cross-sections of the probability density that are color-coded (black represents zero density and white represents the highest density). The angular momentum (orbital) quantum number "ℓ" is denoted in each column, using the usual spectroscopic letter code ("s" means "ℓ" = 0, "p" means "ℓ" = 1, "d" means "ℓ" = 2). The main (principal) quantum number "n" (= 1, 2, 3, ...) is marked to the right of each row. For all pictures the magnetic quantum number "m" has been set to 0, and the cross-sectional plane is the "xz"-plane ("z" is the vertical axis). The probability density in three-dimensional space is obtained by rotating the one shown here around the "z"-axis.