With one "s", as in "vision", the "si" forms the voiced sound sometimes shown as "zh" or as "Z". With the double "ss", as in "impression", the "ssi" forms the unvoiced sound usually shown as "sh" or as "S". "vision" = /v i Z^n/ "impression" = /impr e S^n/ /^/ represents any of several indeterminate central vowel sounds which typically occur in unstressed syllables in English.
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