The ogham alphabet originally consisted of twenty distinct characters (feda), arranged into four family sets or aicmí . A fifth set containing five (sometimes six) additional letters known as the forfeda (sing. forfid) was added in the later Medieval period, bringing the alphabet up to a full 25-character set.
The name forfeda derives from fid (“wood”, a term also used for Ogham letters) and the pr efix for- (“additional”) and they mainly appear in the scholarly tradition, indicating they are a later addition after the peak of ogham usage to cover additional sounds which may have been missing from the original alphabeth.
The use of the forfeda in divination varies depending on author, praxis etc and in many cases are entirely excluded. In some cases they are included as symbols upon a Fionn’s Window/Shield and are used to bring a deeper dimension to its use as a casting surface but they are not part of the ogham set itself. In other cases the forfeda are included in the casting set and given individual divinatory meanings and associations.
Part of the rational for leaving the forfeda out of a set or surface (in my opinion at least) is that there there is no clear consistency in the names and associations applied to the individual symbols. I haven’t really dived deep enough into the matter to identify where the variance comes from but suspect that it is largely based on particular authors and their interpretation of research and their individual personal praxis. As a result I found it very difficult to put together a coherent set of interpretations for the forfeda and had to spend some time reconciling whether or not two authors were talking about the same character by a different name.
This experience is not uncommon. Taking Norse Runes as an example, there are multiple variations of the futhark including (but not limited to) the Elder, Younger, Anglo-Saxon and Icelandic variations. They are contain the same or virtually identical symbols with the same or similar names and meanings but in the case of the runes they have received more attention from academic researchers and pagan authors leading to a greater sense of consensus of which is what. Unfortunately the same cannot be said of Ogham.
As a result I have felt it necessary to preface the next portion of my posts on the Ogham with a “mileage may vary” warning. The meanings and associations I am ascribing to the forfeda are based on my individual interpretations in which I have attempted to avoid duplicating meaning and associations across the original 20 character. As a result it is likely that my posts will not match received wisdom found in books and on other websites but hopefully they will enable you to navigate your own way through the forfeda.