Construction of the Ogham
Ogham writing is unique in that it is a vertical method of writing, starting from the bottom-up. Letters are constructed on a vertical baseline. There are five groups of five letters, each consisting of hash marks on either side of the baseline. Think of the baseline as the trunk of a tree, and the hash marks as its branches.
The original ogham alphabet, consisting of 20 letters, was known as the feda. It was arranged in groups of five letters each. These groups were called aicmí (the singular is aicme). The word aicme means “family.” Each aicme is named after its first letter. The first aicme is Aicme Beithe, the second is Aicme h’Úatha, and so on. Five additional letters were later added because in the original ogham there weren’t enough letters to cover all possible spellings of some Irish words. This fifth aicme is known as the forfeda, or Aicme Koad, for the first character of the aicme.
The Book of Ballymote (Leabhar Bhaile an Mhóta in Irish Gaelic), written circa the 14th century, contains a section known as the Ogham Tract, which explains the ogham in greater detail.