Christopher Masters (2003). Forma. Grove Art Online. Oxford Art Online. Oxford: Oxford University Press. Accessed January 2019. doi:10.1093/gao/9781884446054.article.T028979. (subscription required).
Christopher Masters (2003). Forma. Grove Art Online. Oxford Art Online. Oxford: Oxford University Press. Accessed January 2019. doi:10.1093/gao/9781884446054.article.T028979. (subscription required).
treccani.it
Forma (in Italian). Enciclopedie Online. Rome: Istituto dell'Enciclopedia Italiana. Accessed January 2019.