This kana is derived from 『つ』 (tsu) with a dakuten (voicing mark). In modern standard Japanese, it is pronounced identically to 『ず』 (zu) from 『す』 (su). It occurs in specific native words, often due to rendaku (sequential voicing) or historical sound shifts. For example: 『つづく』 (tsuzuku, 'to continue') where it appears in verb conjugations. Learners should note that 『ず』 is more common, but this form is still used in certain vocabulary.