The title says it all: "Thirteen" is a vamp written in 13/16. Frank Zappa often wrote such odd riffs to be used as backgrounds for guitar solos or while he was addressing the audience. Other examples from the same period can be found on Joe's Garage ("On the Bus," "Catholic Girls"). This particular piece was written before a concert in New York City, October 1978, with Shankar appearing as a guest, and was repeated a few days later on Halloween night. It makes a nice feature for the violinist. It is also worth noting that Eric Dolphy used to perform a song titled "Thirteen" (see "The Eric Dolphy Memorial Barbecue" for the Dolphy/Zappa connection). The piece can be found on You Can't Do That on Stage Anymore, Vol. 6.