Types: Mascaroni
Decorative keystones, or mascaroni, play both an artistic and structural role in the buildings to which they are attached. They contribute to both the art and architecture of Venice. A keystone finishes an arch; it is the last stone to be placed, making the arch strong and increasing its capability to support weight. As the visual center of an arch, a keystone is an integral element in the aesthetic design of a structure and connects the arch with the horizontal moldings that run above it. It may project horizontally beyond the rest of the arch, and keystones are often decorated with masks or figures. The heads and coats of arms that decorate keystones have artistic and historical value, too, but usually fail to attract attention from the public and historical conservation efforts.
Keystones are commonly made from Istria stone, which is hard, waterproof, and easily workable. Its unique characteristics are the reason that much of Venice's public art remains in good condition (and in many cases still legible) today. Keystones are located on bridges, doors, and windows throughout all the sestieri of Venice. In 1995, WPI students documented and photographed 307 keystones in Venice. Forty-three percent of the total arches catalogued are doors, 29 percent are bridges, and 27 percent are windows. The remaining one percent consists of tunnels and one unusual piece – a decorative keystone head on an opening that once housed another piece of outdoor art.
The distributions of the different kinds of arches in which keystones appear are quite even. Doors account for the largest portion of the total number. The number of decorative keystones on bridges is relatively small, however, accounting for only 88 keystones, on 78 out of over 400 bridges in Venice. It was determined that most of the bridges studied had only one decorative keystone.
Stemmi (coats of arms) are the most common decoration for keystones on Venetian bridges. Eighty-seven of the 88 bridges evaluated have stemmi as a keystone decoration. Only a few stemmi are found on doors and other arches. Only one bridge contains a head as a decorative keystone. The rest of the heads can be found on doors, windows, and other types of arches. Doors have the greatest number of heads as keystone decoration. 
Together, decorative keystones on the windows and doors were all heads with exceptions of four stemmi on doors. Doors and windows are often decorated with keystone heads, usually somewhat grotesque, to drive away evil spirits as well as potential human intruders.
Decorative keystones are also subject to damage. Of the total 307 pieces that were evaluated, the most common type of damage was a feature missing from a stemma. Forty-five stemmi pieces were found with at least some part of the shield missing. Some heads were also missing either the nose or the hair, and a few did not have any eyes.
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