The state shares many symbols with the island of Ireland. These include the colours green and blue, animals such as the Irish wolfhound and stags, structures such as round towers and celtic crosses, and designs such as Celtic knots and spirals. The shamrock, a type of clover, has been a national symbol of Ireland since the 17th century when it became customary to wear it as a symbol on St. Patrick's Day. These symbols are used by state institutions as well as private bodies in the Republic of Ireland.