Sushi or Zushi ?

Sushi and zushi:

It comes the way how people describe the word 'sushi' ( 鮨 / 寿司 ) whether phonetically transliterate exactly equall to the Japanese reading and sound, or understandable, comprehensive to others who do not know Japanese well, i.e. English speaking people.

There is a unique rule in Japanese that a word 'sushi' changes its sound to 'zushi' (an unvoiced consonant s changes to voiced z) when it makes a compound word with a preceding adjectival modifier word. Apparently, sushi [ すし ]  →  zushi [ ずし], or su [ す ]  →  zu [ ず ], by an addition of [  ゛], voice-mark to the Japanese kana-character that corresponds to its reading sound.

In general in a compound word, if the latter word begins with a consonant (either of /k/, /s/, /t/, /h/, /f/, /ch/, /sh/, /ts/) each of them changes as follows.

For example, almost all sushi terms that end with suffixed-sushi, this rule is applied (cf. Sushi Glossary):

Other examples:
Also, names of sushi bars and restaurants:


This rule is grammatically called Rendaku.

Wikipedia: Ren_daku (連濁[れん_だく], lit. "sequential_voicing") is a phenomenon in Japanese morphology which governs the voicing of the initial consonant of the non-initial portion of a compound or prefixed word. Rendaku is a common but unpredictable phenomenon in modern Japanese. The "voicing" is not a strict change from voiceless to voiced sounds, but rather the action of adding a 'daku_ten' ( ◌゛: voiced_sound-mark, 濁点) to the first kana of the portion being altered. It is also known as "sequential voicing." Rendaku can be seen in the following: 人 [hito, ひと]  + 人 [hito, ひと] → 人々 (人人) [hito_bito, ひと_びと] ("person"  +  "person"  →  "people") This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Rendaku".

When you are in Japan, or living in Japan, you'd better to use -zushi terms, if you are interested in being regarded as one of sushi fleaks, or not a novice at a sushi bar or restaurant.