Zipf’s law is a succinct yet powerful mathematical law in linguistics. However the mean-ingfulness and units of the law have remained controversial. The current study usesonline video comments call “danmu comment” to investigate these two questions. Theresults are consistent with previous studies arguing Zipf’s law is subject to topical coher-ence. Specifically it is found that danmu comments sampled from a single video followZipf’s law better than danmu comments sampled from a collection of videos. The resultsalso suggest the existence of multiple units of Zipf’s law. When different units includingwords n-grams and danmu comments are compared both words and danmu commentsobey Zipf’s law and words may be a better fit. The issues of combined n-grams in the literature are also discussed.