SANTA CLARA – When Justin Smith was a member of the Cincinnati Bengals, he was the one Bengals defender that offensive coaches had to account for on any given play.

Then again, he wasn't hard to find.

"You knew where he was going to be," said Tony Nathan, the former running backs coach for the Baltimore Ravens who now has the same job with the 49ers.

"We definitely had to make sure our guy wasn't going one-on-one with him all the time. But we knew he was going to be on our left side all day."

With Jonathan Ogden at left tackle for Baltimore, the Ravens usually were able to contain Smith, who almost exclusively played right defensive end with the Bengals. During the two years Nathan was in Baltimore, Smith averaged four tackles a game against the Ravens and registered one sack.

Smith should be harder to locate this season.

On one play during training camp, the 49ers' $45 million free-agent acquisition lined up at his familiar right defensive end spot. On the next, he stood up and played linebacker. Depending on the situation, he also can be a defensive tackle or put his hand on the ground at left defensive end.

More than any other player, Smith is the key to the 49ers' chameleon defense.

Asked recently which scheme – 3-4 or 4-3 – is San Francisco's base defense, coach Mike Nolan said he couldn't choose.

"To be honest with you, they're both right and wrong (because) there hasn't been a definition for (our defense)," Nolan said. "You could say it's a hybrid 3-4 defense. That would probably be the most accurate way to say it."

Smith is the son of a cattle rancher. He's the type of guy who feels comfortable in cowboy boots and a cowboy hat. He sloughs off sensationalism and rejects the notion that he is a 285-pound amoeba, capable of instantly changing his form.

"The only difference between a three-point (stance) and a two-point is before the ball's snapped," he said. "That's really not a transition at all."

But standing up as an outside linebacker does have its advantages. For one, it gives Smith a better vantage point for snuffing out running plays. It also gives him the opportunity to run from one side of the formation to the other before the ball is snapped, something the 49ers hope will keep offenses off balance this season.

When the 49ers first assembled in May, Smith only took a few snaps at outside linebacker.

From that point, however, he's been adding steadily to his repertoire, and coaches have been impressed with how much he can handle.

Nolan said he was surprised that Smith actually had some coverage skills. During Sunday's practice, Smith showed he had zip, coming across the field to run down tight end Delanie Walker and then finishing the play with a hard shove in the back. The gesture turned out to be too hard for the non-contact practice, and Smith nearly got in a fight with resident pugilist Vernon Davis because of it.

Smith's main role, however, will be attacking the line of scrimmage.

The 49ers haven't had a defender finish a season with double-digit sacks since Andre Carter had 12 in 2002. The hope is that by sending Smith from every corner of the field this season, he'll at least come close.

As for Smith, he's not one to make grandiose predictions. But the former Bengal said he does feel as if he's been sprung loose.

"That's really why I came here," he said. "I'm as excited as I've been in a long time."

Read Matthew Barrows' 49ers blog at www.sacbee.com/blogs.