What if we let '_' go, and use 'Q' instead?
I do understand why people would like to use '_', but I guess having a 'Q' is way more important than having a space.
The "_" (space) character is not a actual letter, it just hides the actor that is showing the letter.
There were only 25 frames available, and the alphabet has 26 letters, thus the lack of one letter, in this case "Q"
