Marvin Gaye had enjoyed some duet success via his work with Kim Weston on "It Take Two" but he took the soul duet style to its greatest heights on the albums he recorded with Tammi Terrell. This historic pairing enjoyed a hit right out of the box with their debut single "Ain't No Mountain High Enough," a crossover hit that became one of the 1960's most enduring soul classics. The lyrics present a promise between two lovers who keep their separation from diminishing the feelings they have for each other by promising to always be there in a time of need: "Ain't no mountain high enough/Ain't no valley low enough/Ain't no river wide enough/To keep me from getting to you, babe." The melody lives up to the rousing quality of these sentiments by pairing soaring verse melodies that convey the sweep of the narrators' emotions with a grand-standing chorus that underline their mutual promises with a heroic sense of punch. Gaye and Terrell's recording is a classic Motown combination of pop sweetness and soulful fire: the stately strings give the song an ear-catching sweetness but the hard-hitting drumwork and the pulsing, staccato bassline maintain a quick, driving pace. However, the true hook of "Ain't No Mountain High Enough" is the impressive chemistry between Gaye and Terrell: she balances his gospel grit during the verses with a sweet alto tone but he steps back to let her voice soar over his during the show-stopping chorus. The way their leads intertwine is nothing short of magical so it was no surprise when "Ain't No Mountain High Enough" became a big hit, going top-five on the R&B charts and top-20 on the pop charts. It also inspired a solo cover by Diana Ross that reworked the song into the soul music equivalent of an aria - this epic rendition was a major hit but the Gaye/Terrell version remains the biggest favorite with many listeners. "Ain't No Mountain High Enough" continues to be a staple of oldies radio today and recently made an appearance on the soundtrack of the film Stepmom, where it was treated to an energetic lip-synching by Susan Sarandon.