Columbia subject headings:
Harvard subject headings:
Leaning Tower (Pisa, Italy)
Pisa (Italy)--Buildings, structures, etc.--Conservation and restoration.
Towers--Conservation and restoration--Italy--Pisa.
Princeton subject headings:
Bell towers--Conservation and restoration--Italy--Pisa.
Leaning Tower (Pisa, Italy)
Pisa (Italy)--Buildings, structures, etc.