User dcrockett17 emailed me this handy image:

Now we come to my favorite position on the defensive line, the left defensive tackle. Commonly referred to as a "3 technique" defensive tackle, the left defensive tackle is the tackle equivalent of the right defensive end. It is the primary playmaking defensive tackle. Though it is very rare for a left defensive tackle to record the kind of sack totals of a defensive end, they are superstar non-stat contributors: redirecting rushers, drawing blocking backs, creating fast arriving inside pressure, blocking pass lanes, forcing checkdowns and forcing incompletes.

Glossary:

3 Technique or 3 Tech: Typically the left defensive tackle. The 3 technique tackle almost exclusively plays a single gap, the B, 3, 4 or 3-4 gaps between the offensive guard and offensive tackle. The term is used interchangeably meaning either the position itself, or the typical traits of the position: fast first step, ability to shed blockers, ability to make plays behind the line of scrimmage, ability to create inside pressure and ability to sack the quarterback.

A great left defensive tackle must be nearly as strong as a right defensive tackle and nearly as quick as a defensive end. That’s a demanding and scarce combination. In the past ten years, the position has suffered more than its share of first round flameouts. Players like Reggie McGrew, Wendell Bryant, Ryan Sims, William Joseph and Travis Johnson earned first round selections before doing nearly nothing as pros.

Ever since Mean Joe Greene provided the foundation for the Steel Curtain Defense, teams have sought great, disruptive interior linemen. But the time of the superstar 3 tech tackle may be coming to a close. The rise of the 3-4 is not simply a matter of teams copycatting each other, but the success 3-4 teams have enjoyed. Though many argue it is the versatility of the 3-4 that makes it so successful, I think it’s more likely that the personnel needed to run a successful 3-4 simply isn’t as scarce. Primary pass rushing linebackers, like Shawne Merriman, Terrell Suggs or DeMarcus Ware seem to get up to NFL speed in a flash. Few 3-4 linebackers bust. And many linebackers, like James Farrior, Mike Vrabel, Teddy Bruschi or Junior Seau experience a career revival or renaissance upon transitioning to a 3-4. The Tampa 2 is not complex or overly versatile, but like the 3-4, because the minority of teams employ it, the talent base is richer and the demand smaller.

Prototypical Left Defensive Tackle: Mean Joe Greene, one of the 10 best to ever play.