In the history of early Italian fountains the Fonte Gaia stands alone, with a form and decorative programme that are unprecedented. Compared to earlier extant fountains – the centrally planned, circular basins seen in medieval examples from Siena and across Italy – the Fonte Gaia embodies an entirely different set of formal characteristics. Its unusual arrangement – a roughly rectangular basin open to the sky and walled on three sides – was designed specifically for the sloping topography of the city's central piazza.

Such an innovative expression of civic design raises a number of questions. Why, for example, was the u-shaped configuration chosen, as opposed to the more standard circular basin? Furthermore, how did this unique fountain function for the population that used it? Beyond design considerations are even more basic questions, such as what led to the fountain's construction despite the presence of other, earlier fountains in the city?

This chapter answers these questions through a close analysis of contemporary documents and contextual information. Our conclusions are based on the most important relevant document – the contract for Jacopo della Quercia's fountain – considered within the larger context of Sienese myth-making and artistic patronage in the early Quattrocento. In light of the available evidence, we argue that della Quercia's fountain was far more dependent on the first Fonte Gaia than has previously been recognized, both in form and iconography. In addition, we discuss why the first fountain, completed in 1342, was replaced less than a century after its construction. We then summarize the complex, decade-long evolution of della Quercia's work on the Fonte Gaia. Though the original contract of 5 December 1408 is now lost, surviving copies from 1412 and 1413, as well as drawings, indicate that the plan was modified at least three times before the fountain's final installation in 1419. This chapter also considers the fountain's unusual iconography, synthesizing the work of previous scholars and offering new interpretations where necessary.

While there is scholarly consensus on the identification of the majority of the relief sculptures, the same cannot be said for the two sculptural groups located on the fountain's parapets.