Were letters just randomly assigned to playing cards?
No, the Parlay deck has been carefully balanced in terms of letter value, as well as consonant and vowel distribution. The higher rank poker cards (Ace, King, Queen, etc) have overall lower value letters (like E, N, A), and the lower rank poker cards (Deuce, Three, Four, etc) have overall higher value letters (like Qu, J, W). The four suits have also been balanced in terms of point values and letter distribution.
