A plumb line used to establish true verticals and to position sculpture accurately in relation to surrounding space.
Filo a piombo is the Italian term for a plumb line. Le voci del marmo notes that it is used to find precise vertical lines and to establish the position of sculpture relative to its placement in space.
Etymology
Italian: filo, “line,” + piombo, “lead,” from the traditional lead weight.
Sculptor Notes
For monumental work, vertical alignment affects far more than installation. A figure that is slightly out of plumb can appear unstable even when its internal anatomy and pose are correct.
References
https://levocidelmarmo.sns.it/scheda_oggetti_relazione.php?denom=filo+a+piombo&id=32