f 3743 : y i = VB and x i = back f 156 : y i = VB and y i−1 = MD f 99732 : y i = VB and x i−1 = will and x i+2 = bill It's also important to have features that help with unknown words. One of the most important is word shape features, which represent the abstract letter pattern word shape of the word by mapping lower-case letters to 'x', upper-case to 'X', numbers to 'd', and retaining punctuation. Thus for example I.M.F would map to X.X.X. and DC10-30 would map to XXdd-dd. A second class of shorter word shape features is also used. In these features consecutive character types are removed, so words in all caps map to X, words with initial-caps map to Xx, DC10-30 would be mapped to Xd-d but I.M.F would still map to X.X.X. Prefix and suffix features are also useful. In summary, here are some sample feature templates that help with unknown words: