By Tom Withers

The Associated Press

CLEVELAND Back and forth, side to side, the darting eyes of an NFL quarterback survey everything in their path. And during his 11 years in the league, Trent Dilfer has seen every defense imaginable.

There was Buddy Ryan’#39;s famous ’46.’ Then the ‘Flex,’ ‘Zone-Dog,’ ‘Cover-2,’ ‘Nickel,’ ‘Dime’ and assorted other gimmicky packages.

Dilfer can draw them up on a chalkboard in his mind or rattle off their coach-coded names like they were teammates.

‘I’#39;ve been through it all,’ Dilfer said with a chuckle. ‘When I first came in, the 46 was just going out. It was in vogue for a while. Then the 49ers played a Pete Carroll, George Seifert 4-3, cover-2, zone-dog thing. Then Dallas played a 4-3, cover-4 blanket-coverage thing, so everyone played that.

‘Then Tampa got hot, so everyone switched to their defense. Now New England is hot, and everyone is going to a 3-4. Who knows what they’#39;ll try next. It’#39;s a copycat league, and it will be that way forever.’

Defenses are undergoing another metamorphosis in 2005 as four teams Dallas, Miami, San Francisco and Cleveland switch to a base 3-4 defense, bringing the number of teams using the three linemen-four linebacker alignment to nine, the most since the formation was introduced in the 1970s.

Pittsburgh, San Diego, Houston, Oakland and New England all have been primarily running the 3-4, and almost every team has some modified version in its playbook.

The defense’#39;s renewed popularity is directly tied to the success of the Patriots, who have won three of the last four Super Bowls using the 3-4. Another reason for its spread is that coaches who used it as defensive assistants or coordinators have been promoted to head coaching positions.

‘It’#39;s not so much that

teams are changing,’ said Dallas Cowboys coach Bill Parcells, who won his first Super Bowl title in 1991 using the 3-4. ‘It’#39;s the people that have become decision makers have backgrounds in that defense and that is what they are implementing.’

Follow the bouncing ball.

Patriots coach Bill Belichick first learned the 3-4 while working under Parcells. Browns coach Romeo Crennel, New England’#39;s defensive coordinator the past three years, was an assistant on the same Giants staff. Miami’#39;s Nick Saban also worked with Belichick … and on down the coaching tree.

So why switch to the 3-4?

Proponents suggest it’#39;s most effective in combating today’#39;s sophisticated passing attacks, with their multiple formations and intricate routes, because a fourth linebacker allows a defense the luxury of disguising its blitzes and coverages better.

If the offensive guards are responsible for blocking the inside linebackers, the center has the nose tackle and the offensive tackles are designated to block the defensive ends, who’#39;s got the two outside linebackers? And where are they?

The 3-4 also can morph into a 4-3 in the blink of an eye. The 3-4’#39;s outside linebacker can quickly come up to the line of scrimmage, drop his hand down, and voila, 4-3. Or he can stay on the edge as a decoy, pick up a running back in the flat or cover the tight end.

‘I’#39;m not saying it’#39;s better,’ Parcells said. ‘Here’#39;s what I think some of the advantages are: At any time you can drop eight people into coverage, and that complicates things for the quarterback. Not many four-man line teams have the ability to do that.

‘The second thing is it’#39;s more 1-on-1 football. It forces the offense to block more straight and use less angles than a gap defense does. Sometimes that’#39;s more difficult for the offense to do.’

Dilfer has started 96 games and won a Super Bowl with Baltimore in 2000.

‘For young quarterbacks, it’#39;s hard to read,’ Dilfer said. ‘More than anything else, the 3-4 can take away the rhythm of a passing game. In the 3-4, linebackers can jam receivers and still rush, which takes away a quarterback being able to throw off rhythm. And most of us are not good enough to throw when we’#39;re out of rhythm.’

For the newest 3-4 teams, training camp was devoted to installing the schemes. Some players caught on quickly, others are still learning.

‘We’#39;re taking baby steps with it right now, but by the time the season rolls around, we’#39;re going to be grown men in the system,’ said 49ers two-time Pro Bowler Julian Peterson. ‘I love the new system, but I’#39;m a linebacker, so of course I’#39;m going to love being the guy. This is a great system to build around. Defenses these days have to be fast and furious.’