Not sure why "jam" came to have a musical connotation with extended improvisation and ad-lib structural spins and tangents.

"Let's jam, dudes!" cries the young Dead or Phish enthusiast, strapping on his Jerry Garcia-style Gibson SG and swinging into a three-chord meditation that will last several hours.

Why did he not exhort his fellow musicians by saying, "Fellas, let's marmalade!" or "Let's jelly, my brothers!" or "Turn it up and let's preserves!"?

I don't know.

Perhaps an epiphany will occur to me — or you — Friday and Saturday at the Mystic River Jam. Know this: It's a solid lineup of local and regional acts eminently capable of excellent musical interplay and exploration.

The schedule:

Friday — The Troublemakers (6-7:45 p.m.) and Mystic Dead (8-10 p.m.)

Saturday — Steve Rodgers (1-3 p.m.), Glaucoma Suspect (3-4:45 p.m.), No Mind (5-6:45 p.m.), Jabbawaukee (7-8:30 p.m.) and Wooly Mammoth (8:45-10 p.m.)

The Twins BBQ food truck will be on hand, and alcoholic beverages are available for sale on-site.

Mystic River Jam, 6-10 p.m. Fri. and 1-10 p.m. Sat., Mystic Shipyard, 100 Essex St., Mystic; $10 Fri., $15 Sat., $20 weekend pass, free for kids under 12; mysticriverjam.com.