Does the failure of others to comply with collective duties create duties for us to step in and do their share? Defenders of the so-called duty to take up the slack answer this question in the positive. Against their view, defenders of fair shares argue that we only have a duty to contribute our fair share to discharging the collective duty. This paper offers a partial vindication of Liam Murphy’s account of fair shares. I argue that three common objections to fair shares fail to rebut Murphy’s account. On the reading I propose, fair shares may ground as – or more – extensive duties towards third parties as the duty to take up the slack. In addition, fair shares provides something that defenses of slack-taking tend to omit – an account of the limits of our nonideal duties. The paper highlights previously overlooked aspects of Murphy’s account and paves the way for future discussions about slack-taking in the domain of global politics.