The domes of many trulli are painted with different symbols of mostly apotropaic (= defense against evil) nature. Some of these symbols go back to antiquity but most used today are of Christian religious nature. According to Notarnicola (G. Notarnicola, I Trulli di Alberobello dalla Preistoria al Presente, Roma 1940.), the symbols can be divided into different groups: primitive, magic, pagan, Christian, and ornamental/grotesque. The classification seems a bit problematic to me, but as it is still in use, I will keep referring to it.
The so-called primitive signs go back to a pre-Roman era and can be found on different object groups in the archaeological record of the region. They are mainly simple geomentric patterns of circles, lines and dots, and are rarely to be found on trulli today. The swastica symbol has disappeared, for obvious reasons, as well.
Some symbols from this group have undergone a Chistian re-interpretation, such as triangles for the trinity or a circle for god, or simply by adding a cross to it.
The group of magic symbols consists mainly of zodiac and planetary symbols, as well as the sun and the moon. Sun and moon are sometimes interpreted as representations of the godly and the human nature. Also here crosses are sometimes added.
Pagan symbols from the Ancient Roman era include, according to the beforementioned classification, an eagle, a horse head, a dog or a rooster. I have, honestly, never seen anything like it on a trullo.
Christian symbols are the most common group and are still frequently seen. They evolve around the themes of the cross, different trinity symbols, Christ’s monogram, an orant, or a symbol for Mary. The Jewish menorah (seven-arm candelabra) can be found as well.
The last group, ornamental/grotesque, is more individual. It can consist of the owner’s initials and a sign or object that represents him, i.e. a professional tool, wine grapes, etc. A modern example is, for exampe, the anarchy sign.