[header] How to tune a unison on a piano [title] Open up the piano to expose both the tuning pins and the keys. [step] Depending on the piano, this usually requires removal of the front part of the upper cabinet and sometimes the key cover. [title] Play the offending note and find the strings associated with that note. 
[title] Hold down the key and use a guitar pick to gently pluck each of the three strings that make up the note. [step] Listen for which one sounds different from the others.