The co-volume (in logarithmic space) of the units of a number field is essentially the regulator of the number field. Subgroups of the unit group also lead to interesting co-volumes. For example, the relative regulator of a number field extension L/K arises in this way, from the subgroup of relative units. Eduardo Friedman and Nils-Peter Skoruppa demonstrated an analytic method to prove lower bounds for relative regulators; these bounds grow exponentially in [L:K]. I will discuss joint research with Friedman and Chinburg to generalize this method to other subgroups.