In https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.455.9354&rep=rep1&type=pdf it classifies letters into five groups: - vowels `A E I O U` - common consonants `T N S R H` - secondary consonants `L D C P H` - tertiary consonants `M W Y P G` - high scrabble value letter `J K Q V X Z`