As we can see from the partial POS lexicon for English in Figure 1, "the" is unambiguous with respect to POS tagging, since it can only be a determiner (DT), whereas "sting" is ambiguous, since it can be a common noun (NN), a proper noun (NNP) or a verb (VB).
the lexicon imposes constraints on the possible POS tags  Conceivably, tagging accuracy decreases with the increase in ambiguity: unambiguous words such as "the" will always be tagged correctly; on the other hand, unseen words (or words not present in the POS lexicon) are among the most ambiguous words, since they are not constrained at all and therefore can receive any of the POS tags.