A dictionary for compressing and decompressing textual data has a number of keys. Each key is associated with an identifier. The keys include static word or phrase keys, where each static word or phrase key lists one or more unchanging words in a particular order. The keys further include dynamic phrase keys, where each dynamic phrase key lists a number of words and one or more placeholders in a particular order, and each placeholder denotes a place where a word or phrase other than the words of the dynamic phrase key is to be inserted. At least one of the dynamic phrase keys may identify one or more of the words by identifiers for corresponding static words or phrase keys. At least one of the static word or phrase keys may identify one or more of the words by identifiers for corresponding other static words or phrase keys.