Players take turns forming words on a grid. Words can be formed horizontally or vertically, as in Scrabble, but, as the title suggests, the letters may also be stacked. This lets already played words change into different words by stacking new letters (up to a limit of 5 high). For example, LATE might be changed to CATER and then to BELATED. Instead of having different values for each letter, when a new word is made, the number of tiles used in that word is counted. If the whole word is one tile high (no stacking), then the word scores 2 points for each tile. However, if there are any stacked tiles, you get one point per tile in the word, including those underneath. This results in scoring that increases as the game progresses, even for simpler words.