A kissing cousin of the Sidecar, which forsakes some of the lemon juice for with the addition of rum, resulting in a drink that's little less sophisticated, and a little more dangerous.
Between The Sheets Dates back to prohibition times (when the jaws were squarer and the drinks were stronger), and could be considered to be the great-grandfather of modern saucily-named drinks like the Slippery Nipple or the Screaming Orgasm.