The "up/down" arrows are a bit confusing, I don't think it's self-evident whether "later in the alphabet" would be considered "higher" or "lower".
"left" or "right" might be clearer, at least for English, and it would map directly to the range bar visualizations beneath each letter.
Higher = right = after, Lower = left = before. Once you play some games, it will become more natural. At least this was the feedback from friends playing the game.