Dot-based tactile alphabets are far more readable than other types, because dots maximize the number of broken paper fibers sticking up vertically out of the tops of the paper to tickle the fingertips, which is what Braille readers actually feel when reading - they don't actually try to feel the bumps, they instead feel the much higher frequency stimulation from the raised fibers that reach above the bumps (I apologize for not having a citation on this, I spent a great deal of time 20 years ago researching tactile technology for the blind, and this was a highly surprising and broadly known and reproduced result at the time).