erg second

The erg second is the unit the quantum was born in. Planck's first determination of his constant, published in 1901, came out at $6.55 \times 10^{-27}$ erg seconds, within about one percent of the modern exact value of $6.62607015 \times 10^{-27}$ erg·s, and the constant lived in this unit through most of the twentieth century's physics literature. Reading any quantum mechanics text from before the 1970s means reading action in erg seconds, and the conversion is mercifully clean: shift seven decimal places, since the erg is exactly 10⁻⁷ joules.

Astrophysics never left CGS, so the unit still works for a living there. Stellar and accretion disk angular momenta are quoted in g·cm²/s, which is this same unit under its mass-times-velocity name, and the Sun's rotational angular momentum comes to about $2 \times 10^{48}$ erg·s. The span from there down to the reduced Planck constant at $1.05 \times 10^{-27}$ erg·s is some seventy-five orders of magnitude carried by a single unit, which no SI quantity is routinely asked to do.

On this site the Planck constant and its reduced form sit two entries away in this same catalog, now exact by the 2019 SI redefinition, and the photon energy solver in the modern physics section, $E = hf$, is where the constant this unit once carried does its present-day work.