The 49ers and the Seahawks are competitors on the field, while their fans compete off the field.
A new controversy has arrived at the new doorstep of the San Francisco 49ers. It comes courtesy of the team's competitor to the north.
Some Seattle Seahawks fans want to buy a commemorative brick at the new Levi's Stadium in Santa Clara.
The red and gold bricks cost between $195 and $545 each - and allow fans to write three to six lines of text.
Some Seahawks fans have already raised hundreds of dollars online to inscribe "Go Hawks" on a few bricks.
Another brick would read, "Congrats on the second best stadium in the NFC West."
Stadium officials could end up blocking fans from opposing teams from buying the bricks.
Now - after an uptick in media attention - some Seahawks fans say - they'll probably scrap the trash talk, and go with something more civil.