Half-mass radii of the clusters as a function of their age. The open circles indicate clusters still containing large fractions of their natal gas (values from Lada & Lada 2003), whereas the full circles represent clusters that are largely devoid of gas (values from Pfalzner 2009). The solid, dashed, and dotted lines represent our simulation results of the cluster development after gas expulsion for model clusters A1, A3, and A5, respectively.