Any Catholic priest may
wear a skullcap. This head covering goes by different names. It is sometimes called a solideo, calotte, pilleolus or sub-mitrale.
It is most often spoken of as a zucchetto.
Originally, it had a very practical purpose. Clerics wore a hairstyle called the tonsure.
The hair on the crown of the head was cut and shaved. A skullcap was then often worn
to keep the head warm.
While some priests may continue to use a skullcap for utilitarian reasons, members of
the hierarchy must use the zucchetto as a liturgical vestment. Its color will tell you
the person’s office.
According to James-Charles Noonan, Jr., in The Church Visible (Viking), the zucchetto
did not develop from the Jewish yarmulke, but has a development all its own.