IRVING, Texas -- The Dallas Cowboys are the envy of the National Football League in one way that actually has to do with football: how they have built their offensive line.

In 2011, the Cowboys drafted Tyron Smith in the first round. They took Travis Frederick in the first round in 2013 but were criticized in some circles for taking a player considered a fourth-rounder by some. In 2014, they took Zack Martin in the first round despite Johnny Manziel and all the trappings he would have brought staring them in the face.

Last year the Cowboys had a bit of good fortune, signing a first-round talent, La'el Collins, as an undrafted free agent.

Shrewd moves have allowed the Cowboys to build a formidable offensive line. USA TODAY Sports, Icon Sportswire, USA TODAY Sports

Smith, Frederick and Martin have played together in the last two Pro Bowls. Smith has been to the last three. Collins had a solid rookie season with some highlight plays. Right tackle Doug Free is entering his 10th season, but last year the Cowboys drafted Chaz Green in the third round with the idea he would one day be the starter.

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That's four starters since 2011. Smith is signed through 2023. Frederick will be a Cowboy through at least 2017, Martin through at least 2018 and Collins will only be a restricted free agent after the 2017 season.

All four came from big schools that run pro-style offenses, not the spread-only schemes that roll up points but don’t translate much offensive line play to the NFL.

“It's one of things you see at a number of different positions,” coach Jason Garrett said. “The style of offenses that many of these players have played in in college may be different than what we are going to ask them to do. When you are spread out and paying East and West and not seeing a lot of fronts and you are not asked to drop and pass protect as much as we would ask guys to do, it makes their development and their transition that much more challenging.”

With the fourth overall pick, the Cowboys could be in position to take another offensive lineman in the first round if Laremy Tunsil does not go among the top three picks. Maybe Notre Dame’s Ronnie Stanley could be an option if the Cowboys trade down.

Tunsil is considered by some to have similarities to Smith. However, he might not have the sustained strength like Smith. Stanley is a terrific athlete who could use more strength. Either would make an elite group even better.

But would the Cowboys make such a play?

It’s doubtful, even if you subscribe to the best player available theory.

Making it work against the salary cap in the next year or two would be possible, but with Frederick, Martin and Collins all coming due in the next three seasons, placing that much of a cap expenditure in one position is not pragmatic.

It is easier to do without a quarterback counting more than $20 million against the cap, but the Cowboys have Tony Romo signed through 2019.

This is another reason why the Cowboys have to succeed in Romo’s final years, however long that may be.

When the Cowboys took Martin in the first round, they were compared to the San Francisco 49ers, who played in two conference championships and a Super Bowl from 2011 to 2013.

The Niners had first-round picks up front in Joe Staley, Mike Iupati and Anthony Davis. By 2015, only Staley remained, with Davis retiring and Iupati leaving as a free agent for the Arizona Cardinals. The Niners finished in last place in 2015, fired their coach and have since hired Chip Kelly.

The days of offensive lines growing older together, like what happened with the Cowboys in the 1970s and '90s, are over, mostly due to the salary cap. The New England Patriots used 13 different starting offensive line combinations last year.

The Cowboys had three with Ronald Leary first getting replaced by Mackenzy Bernadeau because of injury and later by Collins.

The Cowboys built their line the right way. Now it’s about maintaining the group.

The question now is for how long.