In the tutorial, it is said that:

Let v range over the tags. Define U(k, v) to be the score of the best sequence of tags from 1 to k, where tag k is required to be v. This is a maximization over k −1 tags because tag number k is fixed to have value v

It is very confusing that why should the Kth tag be fixed?

  • 2
    $\begingroup$ v is the STOP tag, and the sequence needs to end with a STOP tag. $\endgroup$ Dec 28, 2016 at 18:04

1 Answer 1


I learned from here that the first and last tokens are all fixed.

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