In the course of studies on the graffiti along the walls of the piano nobile of Palazzo Pubblico in Siena an inscription emerged below the 'Effects of Good Government in the Country', dated 20 April 1409, whose solemn appearance distinguished it from the others. It is in fact a promissio etched onto the wall, preceded by a chrismon as a formal invocation, whose object is the building of the fountain of Piazza del Campo, called “leta” instead of “gaia”. The dating, content and forms of the inscription all converge in suggesting that its author was Jacopo della Quercia, who, as is known, in December 1408 had been commissioned to create a monumental new structure for the fountain, on which he only concretely began work in 1412. Della Quercia held numerous public offices and for several years frequented the Palazzo Comunale, a period that started – not surprisingly – at the end of 1408, and culminated in his election as operaio of the Duomo in February 1435. The graffito in the Sala della Pace not only constitutes an important and previously unpublished testimony to the complex relationship between Siena and the sculptor, but also marks a link between his artistic and political life, through reference to a work – the Fonte Gaia – which can rightly be considered, at least in its intentions, another memorable “effect” of good Sienese government.