At least some content in this article is derived from information featured in Harry Potter: Hogwarts Mystery. Spoilers will be present within the article.
The Transfiguration alphabet is either a subject within Transfiguration or a tool to learn it. Minerva McGonagall's Transfiguration classes in the 1980s involved the Transfiguration alphabet somehow.
The Transfiguration alphabet could be seen on the chalkboard at the back of the Transfiguration Classroom in both the 1980s and 1990s. The board details the translations of each letter, showing that the Transfiguration alphabet consists of 26 characters that directly translate to the letters of the English alphabet.