Passacaglia in d minor (BuxWV 161)

Some say that Buxtehude's Passacaglia influenced the writing of Bach's c minor Passacaglia & Fugue (BWV 582). It's speculation, but in many ways we can say that Buxtehude had considerable influence on Bach.

Buxtehude's mastery (1637–1707) bridges the post-Reformation protestant period of Jan Pieterszoon Sweelinck (1562-1621), Samuel Scheidt (1587-1654) - who also studied with Sweelinck - and the high Baroque of Bach's days (1685-1750). Those who argue that Buxtehude will forever stand in the shadow of Bach need to study their history: the 20-year old Bach himself becomes the student who walks 400kms (and again, for his return journey!) to hear & meet the master:

"It’s autumn 1705 in northern Germany. A young composer sets out from the city of Arnstadt to make the gruelling journey on foot to Lübeck, almost 400 km to the north. His reason? To hear some music by an organist – a 68-year-old organist, in fact, who is nearing the end of his career. But this isn’t just any organist, and the steadfast walker isn’t just any young composer." See https://bit.ly/2WMaRiV

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There are four sections, exploring a total of three keys. The first section is in D minor (the tonic), the second in F major (the relative major), the third in A minor (the dominant), and the fourth returns to D minor. The sections are connected by short modulatory passages. Each section contains seven variations on the seven-note ostinato. Modulation was rarely seen in ostinato variations at the time; nevertheless, an Italian composer of the mid-17th century, Bernardo Storace, used the same scheme in his passacaglias (four sections in different keys, connected by short transitions); but it is unlikely that Buxtehude knew Storace's work.

See https://en.wikipedia.org/wiki/Passacaglia_in_D_minor,_BuxWV_161