Original Ogham alphabet has 20 letters. In the 9th century, 5 more letters were added to denote two vowels side by side – EA, OI, UI, IA, AE. Adapting to meet the modern 26 letter alphabet, some letters are substituted.  J is substituted with I, K with C, V with F, W with U, Y with I, P with Q. Later, new symbols were added for P and K.