Chemistry is a mental thing.
It's trust between the guard positioned just inside the tackle, some unspoken communication at the line of scrimmage and a deep bond off the field.
Or maybe chemistry is a physical thing.
It's the way those players move as one on game day, the techniques honed by pounding on each other after practice and the way that friendship can allow a group of teammates to push every member through workouts.
The Tennessee offensive linemen are always quick to point to chemistry as the reason they appear poised for a breakout season together, though putting a finger on what exactly that means isn't easy for any of them. Regardless of what it is or how the Vols have found it, the five guys settling into starting roles are clearly counting on it having a big impact when they open up the season against Montana at Neyland Stadium on Saturday (TV: pay-per-view, 6 p.m.).
"The biggest thing with the chemistry is building trust," left guard Alex Bullard said. "In some of our protections, we have to know if we have help or not. That's something we need to know, because Dallas (Thomas) needs to trust that I'm going to be there.
"If he doesn't trust that I'm going to be there, that's going to screw up the whole thing. That's the biggest thing, having a feel for each other, trusting each other, knowing how everybody communicates."
Of the five pieces the Vols have turned into one whole, Bullard was the final addition after getting reps all over the line thanks to his versatility. But ultimately UT couldn't justify keeping the transfer from Notre Dame off the field and needed to settle on a rotation to start developing some cohesion across the line, which finding the perfect spot for Bullard allowed it to do for most of training camp leading into preparation for the Grizzlies.
It obviously helped that UT had four returning starters in place, with Ja'Wuan James and Zach Fulton holding down the right side, James Stone at center and Thomas at the critical left tackle position.
And while teamwork and communication are important at any position, perhaps nothing requires as much singular focus or unified effort as playing in the trenches — which made finding a fifth starter early in camp critical in the unscientific search for chemistry.
"That's a big deal, stability-wise, knowing those are the five guys and we're set in there to start the season," James said. "We're building trust in there. Alex can do that with Dallas since Dallas has gone through a lot of left guards on that side and has tried to build chemistry with all of them.
"I mean, I have a good chemistry with Zach and I trust Zach on the help-side. We know in games, we're not going to let a defensive end penetrate on each other. At the same time, when I'm helping Zach, he needs to know in the back of his mind that he doesn't have to worry about me and he can focus on his job. Same for me, I can focus on my job knowing Zach will be there for me."
On the other side of Fulton, the same is true for Stone.
Stone then needs to have the same level of confidence in Bullard, who is then leaning on Thomas.
The rest of the offense is counting on all those guys to deliver as well, with a running game that ranked last in the SEC last year looking for bigger holes and quarterback Tyler Bray banking on the total of sacks dropping from 41 a year ago.
Starting against Montana, that progress will be easy to monitor on the stats sheet. Just how the linemen produce it might be up to a bit of interpretation.
"Me and Zach, we've been playing together for a long time now it feels like, so we're doing pretty good chemistry-wise," James said. "We make calls and stuff like that, but normally with me and Zach, sometimes we don't really have to make the calls.
"Sometimes we just say, 'You already know.' "
Like chemistry, the Vols don't need to put everything in words.