Computing within Limits invites considerations of limits andconstraints in design practice. We compare two projects whichintegrate constraints, the reduction of academic air travel and asolar powered internet, to show a distinction between twoapproaches to arriving at constraints. In the case of reducingacademic air travel, the problem which greenhouse gas emissionspose for business-as-usual academic travel is addressed byproposing constraints on future flying. Constraints in the Flightproject can be understood as a process of commensuration, ofcomparing that which is to be constrained according to a commonmetric. This gives rise to a future of academic travel understood inrelation to CO2 emissions and reduction targets. In the secondcase, we have explored the solar internet as a specific way tointroduce constraints in the context of the rising electricity useassociated with internet infrastructure. In the Solar Internetproject, constraints have been approached relationally anditeratively, in reconfigurations of internet use practices and designpractices, including the solar internet imaginary and the scale ofbattery and power supply.

We compare these two approaches, drawing on vocabulary fromSociology of Quantification and Science and Technology Studies,to help articulate their respective implications, while alsoacknowledging what they have in common, e.g. the ability toexpand the frame of what is made relevant for design practice.The case of the Flight project suggests that constraints as aprocess of commensuration can be fruitful when pursuing aunified future, intervening over time with a trajectory towards aquantifiable target. On the other hand, when trying to account forindirect effects and the future as multiple, the introduction ofconstraints can better be understood as con-figurations, with afuture negotiated iteratively in design practice. Rather thanthinking about constraints as essentially requiring one or the otherapproach, we suggest that problems and the introduction ofconstraints may be more or less amenable to either approach at aspecific time.