Ogham is an ancient writing system used primarily in Ireland and parts of Britain, dating back to the 4th to 6th centuries AD. It consists of a series of lines and notches carved into stone or wood, representing letters. Each letter is associated with a tree or plant, reflecting the importance of nature in Celtic culture. The Ogham alphabet contains 20 characters, known as "letters," which are grouped into four sets of five. This unique script was often used for inscriptions on grave markers and boundary stones, serving both practical and ceremonial purposes in early Irish society.