Ella Fitzgerald was early in her historic run on Norman Granz’s Verve label when she was paired up with the great Louis Armstrong in the summer of 1956 for Ella and Louis. Their Porgy & Bess album followed soon after, as did Ella and Louis Again (all of it collected on The Complete Ella and Louis on Verve). The alchemy between the two giants is a marvel in the annals of American song. They take a simple approach that essentially can’t go wrong: beautiful expressive standards, uncluttered arrangements, virtuoso small-group backing from pianist Oscar Peterson’s working unit with bassist Ray Brown and guitarist Herb Ellis—plus Buddy Rich on drums, as exquisitely delicate as you’ll ever hear him. Taking separate choruses or harmonizing in vivacious and perfectly timed call-and-response (with Armstrong on trumpet and voice), the partners casually arrive at version after definitive version of these adored songs, pouring themselves equally into shimmering ballads (“Moonlight in Vermont,” “Stars Fell on Alabama”) and levitating midtempo swingers (“Can’t We Be Friends,” “Under a Blanket of Blue”). Fitzgerald’s masterful Armstrong impression in the last phrase of “Tenderly” captures the fun of it all.