All 30 Major League Baseball teams in action tonight will are honouring those lost on September 11, 2001 by wearing a United States flag patch on the left sides of their caps.
The Toronto Blue Jays will be honouring both the American and Canadian victims of the attacks by wearing a US Flag on the left side and a Canadian flag on the right side.
In the weeks following September 11, 2001 all MLB teams wore the same patch on their caps (the Blue Jays and Expos both chose to only wear an American flag patch back in 2001) as well as on the backs of their jerseys. Teams did this once again last year for the 10th anniversary.
There are no special jersey patches today.
Also note that regular game caps are being worn, not the “Stars and Stripes” camouflage version typically worn on days such as these.