  Most stars do not form in isolation but as part of a cluster comprising
anywhere between a few dozen to several million stars with stellar densities
ranging from 0.01 to several 10$^5$ \Msun pc$^{-3}$. The majority of these
clusters dissolve within 20 Myr. The general assumption is that clusters are
born more or less over this entire density range. A new analysis of cluster
observations is presented. It demonstrates that, in fact, clustered star
formation works under surprisingly tight constraints with respect to cluster
size and density. The observed multitude of cluster densities simply results
from snapshots of two sequences evolving in time along pre-defined tracks in
the density-radius plane. This implies that the cluster size can actually be
used to determine its age.
