Initial public pensions are indexed to the economy-wide average wages, butpensions in progress are indexed to prices, average wages or their combinations– varying across countries and periods. We create a simple overlapping cohortsframework to study the properties of indexing pensions in progress – emphasizinga neglected issue: close wage paths should imply close benefit paths even at realwage shocks. This robustness criterion of an equitable pension system is onlysatisfied by wage indexing, which in turn requires the adjustment of the accrualrate. To minimize the redistribution from low-earning short-lived citizens tohigh-earning long-lived ones, progression should be introduced.