What was the title of the instrumental Peter Green named after the rhythm section?

Peter Green named the instrumental “Fleetwood Mac” after the rhythm section of Mick Fleetwood and John McVie.

The track emerged from recording sessions made after Green left John Mayall & the Bluesbreakers. Mayall gave Green free studio time, which Green used with Fleetwood and McVie to record five songs. The fifth was an instrumental, and its title combined Fleetwood’s surname with “Mac,” a shortened form of McVie.

The title then became useful beyond the recording itself. Green and Fleetwood wanted to form a new band with McVie, and they called the proposed group Fleetwood Mac partly to persuade the bassist to join. McVie initially declined because he preferred the security of Mayall’s employment, so Bob Brunning temporarily filled the bass role.

This is why “Fleetwood Mac” is both a song title and a band name. It is not a reference to the later hit “The Chain,” the blues track “Long Grey Mare,” or the instrumental “Albatross.”