David Shin, a silver collector, ran short of cash. He borrowed $1,500 from Vinnie Gaff, giving Gaff the following note: “Thirty days from date, I promise to pay to the order of Vinnie Gaff $1,500 worth of silver (signed) David Shin.” Gaff indorsed the note and gave it to Kia Lai in exchange for services rendered by Lai. What legal term describes the transfer of the note from Gaff to Lai? Explain your answer.