Continuity on lines a crucial building block

There will be moving parts aplenty when the Patriots take the practice field this morning for the start of training camp, but it won't be hard to spot the offensive and defensive linemen among the 70-plus players donning helmets. They're the biggest of the bunch, the 300-pounders.

Size alone, however, is not the reason they should stand out. The linemen are in a unique place compared with the rest of the NFL.

Some teams return all starters on the offensive line. Others return all starters on the defensive line. But only eight of the league's 32 teams, including the Patriots, can boast they are returning all starters on both lines.

Only 25 percent of NFL clubs return both starting lines, hardly a surprise in the free agent, salary cap era in which player movement is brisk (126 unrestricted free agents signed with new teams this offseason).

And even when a team thinks it has stability, such as the defending champion Colts, it can be fleeting. Indianapolis was primed to return top players on both lines before veteran left tackle Tarik Glenn, who had started 154 career regular-season games the last 10 years, retired earlier this week.

So it takes fortune to keep offensive and defensive lines together. The free agent cycle is also bound to catch up with every club.

But teams that keep their lines together -- with the right personnel -- enter training camps with a headstart. Division champions San Diego and New Orleans, as well as Carolina, Green Bay, Jacksonville, the New York Jets, and Minnesota also have both lines intact.

"We all know how hard it is for a larger group to work together in maximum efficiency," said Patriots coach Bill Belichick, when informed his team was in the minority by returning top personnel on both lines.

"Certainly on the offensive line, having guys who have played in the system, played with each other, that's a big help. There is always a process where those five guys have to see the same game, the same picture, to better identify it, and that is a lot easier said than done. I'm glad we're in that situation."

How the Patriots put themselves into that situation -- which could also be the case next year -- is a reflection of multiple factors. It starts with good scouting and drafting.

Of the top returning defensive linemen, Richard Seymour (2001), Ty Warren (2003), Vince Wilfork (2004), and Jarvis Green (2002) were all draft picks. The fifth lineman, the emerging Mike Wright, was signed as a rookie free agent in 2005.

It's all home-grown talent on the offensive line as well. Tackles Matt Light (2001), Nick Kaczur (2005), and Ryan O'Callaghan (2006); guard Logan Mankins (2005); and center Dan Koppen (2003) were draft choices, and starting guard Stephen Neal (2001) was signed as a rookie free agent and developed on the practice squad. Other top reserves, Russ Hochstein and Wesley Britt, took the same path.

Simply picking talented players, however, isn't enough. Cultivating them and coaching them is the next step, and few would contest the Patriots' expertise in that area.

Finally, it takes some cold, hard cash to keep it all together. Top talent on the line can command big bucks, and the Patriots reached deals with Seymour, Green, Light, Koppen, and Neal when their initial contracts were set to expire.

As the Patriots hit the practice fields today, look in the far righthand corner, farthest from the bleachers, which is usually where the offensive and defensive linemen run through individual drills directed by assistants Dante Scarnecchia (offensive line) and Pepper Johnson (defensive line).

The linemen won't need a reminder of how things are done.

Mike Reiss can be reached at mreiss@globe.com.