In the arresting, proclamatory opening of Luciano Berio’s Sequenza VIII for violin (1976), a single note is hammered out repeatedly while other notes just above and below clash with it. One of these clashes, A vs. B, becomes the piece’s main motif—or, as the composer put it, “act[s] as a compass in the work’s rather diversified and elaborate itinerary.” For Berio, this two-note underpinning connected the piece to Bach’s chaconne in D minor for solo violin, which also spins virtuoso variations on a repeated foundation. To me, the Sequenza’s most obvious reminiscence is to the early-19th-century Caprices for solo violin by Paganini—especially a long, rapid perpetual-motion passage at the work’s center—and this in turn, going back another century to a third Italian composer, reminds me of the driving agitation of “Summer” from Vivaldi’s “Four Seasons.” By coincidence, the Berio and the Vivaldi, two showpieces from opposite ends of the Italian violin tradition, are being played simultaneously this weekend. (Not in the same venue, sadly, which would be a lot of fun.) Graeme Jennings plays the Sequenza, alongside other solo pieces by Salvatore Sciarrino, Jeremy Jolley, and Franco Donatoni, as the guest of the Seattle Modern Orchestra; while Carrie Krause is the violin soloist in an evening of Vivaldi concertos with the Seattle Baroque Orchestra. SMO: Chapel Performance Space, 4649 Sunnyside Ave., seattlemodernorchestra.org. $10–$20. SBO: Town Hall, 1119 Eighth Ave., earlymusicguild.org. $20–$45. Both 8 p.m. Sat., April 11.