Board Thread:Lore Discussion/@comment-26833545-20150729122320/@comment-6881629-20150729123141

The Greybeards are seen writing with their voice though, literally shouting the words into the stone floor of High Hrothgar. But the dragon alphabet page does say -

"The shape of the letters is constrained by their need to be written using three large talons and a vestigial fourth digit called the dewclaw. Thus, each letter consists of at most three scratches and a dot"

Although theres no source listed to confirm it the letters do look like scratches.