Oh My Darling, Clementine
"Oh My Darling, Clementine" was a traditional 19th century Earth song. The Doctor and Seven of Nine used to practice duets together, including "My Darling Clementine", which he described as an "old chestnut."
 
In 2376, The Doctor and Seven sang lines from the song while he was working on her cortical implant. 

In their "duet", The Doctor and Seven sang:

"Oh, my darling, Oh, my darling, Oh, my darling –"
"Clementine."
"You are lost and gone forever –"
"Dreadful sorry, Clementine."
"Light she was and like a fairy –"
"And her shoes were #9."
"Herring boxes without topses –"
"Sandals were for Clementine."
"Drove she ducklings to the water..."

Following the ordeal, Seven told The Doctor that he was off-key in the third verse, second measure. The Doctor insisted that he had not been, but Seven explained that his vocal modulations deviated by .30 decihertz, which she could assist him with. He agreed, and coordinated a lesson together, with a tuning fork, for the next day. ()

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Oh My Darling Clementine