Models can also handle alphabets other than the simple four-symbol set chosen for this example. More sophisticated models are also possible: "higher-order" modelling changes its estimation of the current probability of a symbol based on the symbols that precede it (the "context"), so that in a model for English text, for example, the percentage chance of "u" would be much higher when it follows a "Q" or a "q". Models can even be "adaptive", so that they continually change their prediction of the data based on what the stream actually contains. The decoder must have the same model as the encoder.