In phonology, the ts–ch merger is the merger of the voiceless alveolar affricate /ts/ and the voiceless postalveolar affricate /tʃ/.
In Russian, it is the merger of the consonants rendered by letters Che and Tse. If the shift is towards Tse, it is called tsokanye (Russian: цоканье); the shift towards Che is called chokanye (Russian: чоканье).
It is a regular sound change of Lower Sorbian, but not Upper Sorbian, as seen in the difference between Lower Sorbian cas and Upper Sorbian čas, both meaning "time".
In Polish language the /tʃ/ → /ts/ merger is part of a more general dialectal feature called Mazurzenie present in many Polish dialects, named for the Masovian dialect.
It also occurs in a few areas of the Chakavian dialect of Croatian, known as tsakavism.
The sabesdiker losn feature of Northeastern Yiddish includes the /tʃ/ → /ts/ merger.