OAKLAND -- Much of Cliff Pennington's childhood was spent on the diamond, in a steamy Texas heat, between second and third base on the left side of the infield.

"I was always a shortstop," Pennington says.

Maybe so, but there was also a time at Mary Carroll High School in Corpus Christi, Texas, when he played second base, due to a rather weak arm.

That same arm ultimately found its way to the mound, a result of his throwing batting practice to the varsity squad, one of Pennington's freshman-year duties.

"I came back the next year pitching for us," Pennington recalls. "My arm got stronger because of it."

The rest is seemingly history, but the cannon that is now Pennington's arm is part of what's transforming this 26-year-old, who stands just 5-foot-11, into one of the game's finest defensive presences.

"He's gotta have one of the top five arms in Major League Baseball," A's infield coach Mike Gallego said. "Not only does he have the strength, but the accuracy he has with that thing is incredible."

"He's tremendous," teammate Mark Ellis said. "He makes the plays in the hole, makes the plays up the middle. He can go both ways. His arm is one of the best I've seen in the big leagues, and he's only going to get better."

Strangely, Pennington's arm has also turned into something of a sweet burden, a gift that has led, in part, to the A's infielder racking up 18 errors, which ranks first among all American League shortstops.

"When you say he has 18 errors, obviously, that's a lot for a shortstop of his capability," Gallego said. "At the same time, I think Cliff Pennington is going to make more errors because of his ability. He's going to get to more balls, and he's going to attempt to throw more because of the arm he has.

"He has a tendency to sit back on a lot of balls, because he can depend on that arm so, in turn, if you sit back, you're going to give the ball a chance to take a bad hop. He does get a lot of in-between hops, but he's got such good hands people don't recognize it. He's got the arm that lets him get away with it, keeps him out of trouble."

So, despite his knack for collecting errors at a pace he'd like to see decrease, Pennington also ranks first among all qualifying shortstops in the league in zone rating, the percentage of balls fielded by a player in his typical defensive "zone," as measured by STATS, Inc.

Furthermore, he ranks third in range factor, which simply calculates the number of plays made per game at the fielding position, taking into account the fielder's own ability to get to a batted ball. By day's end, this statistic ultimately rewards the more talented players at each position, which is why many in the game are now weighing its outcome more than a player's fielding percentage.

"I compare Penny a lot to Troy Tulowitzki," said Gallego, who groomed Tulowitzki during his Rockies coaching days. "He had the same kind of style of sitting back on the ball. Penny's obviously fielded that way since high school, and it got him to the big leagues, so who am I to change that? Because of that, I believe he's not going to be errorless ever at shortstop. He's not going to be flawless, but he's gonna be a thrill to watch."

The defensive style Gallego speaks of isn't the lone reason for Pennington's sudden surge into an exciting everyday player. This Texas kid, a first-round Draft pick by the A's in 2005, has matured in every sense of the word by way of confidence, a part of his game that wasn't necessarily missing, but possibly lacking, when he was initially called up at the end of the 2008 season.

It was then he bounced his way around the infield, making a mark not only at shortstop but third base and second base as well. Triple-A Sacramento proved to be his home at the start of 2009, but following the Orlando Cabrera trade on the last day in July, Pennington was again rewarded a ticket to The Show.

"When you break the club, it's obviously exciting," Pennington said, "but you're still trying to fit in, try to see what it's all about. You have to get used to seeing the game, seeing the guys you grew up watching. As you start to get a few games under your belt, you see that you can do it, that you can play at this level."

Even then, though, following a stretch that saw Pennington start 60 of the club's final 61 games at shortstop, the confidence factor still wasn't in full bloom.

Fast forward to Spring Training this year, and cue in Adam Rosales, the jack-of-all-trades kid acquired from Cincinnati via offseason trade.

"Adam was one of those guys that not only could field pretty well -- [he] had a great arm just like Penny -- but also could hit, hit with some power, brought some energy," Gallego said. "There were a lot of pluses Adam brought to the game. I sat back and watched Penny's reaction to all this and listened. If you sit back and listen to Penny long enough, he's going to tell you how he feels."

Their conversations, Gallego said, brought out the competitor in Pennington, who in time told his coach that the shortstop position was his, and no one was about to take it from him.

"He was kind of joking," Gallego said, "but he was trying to prove a point -- not to me, not to the manager, but to himself. And he succeeded. He started off slow, and then he started swinging the bat, making the plays, and doing the things that we needed and expected of him at shortstop."

Since, Pennington has started 111 of the team's 123 games en route to cementing his name in the everyday lineup, where he's also offered a good dose of punch in the No. 9 hole while providing some speed on the bases with 20 steals, the most by any A's shortstop since Alfredo Griffin tallied 26 in 1987.

With 39 games remaining in the season, that mark can still be reached. Pennington's got time and lots of it, thanks to a somewhat newfound ability to control the pace of the game.

"I've made so many improvements from when I was drafted to when I got here, especially on defense and slowing things down," Pennington said. "A lot of it has come with experience and just trusting myself. It's about the confidence in knowing that I'm going to make the play."

"He's a very confident player," Ellis said. "He stays within himself and really doesn't try to do more than he's capable of doing. As a shortstop, you've got to do all the little things right. You've got to make all the plays. He's done that, and then some."

Ellis' veteran defensive presence at second base, perhaps one of the more underrated in the game, has aided Pennington immensely. In fact, Pennington essentially spent his initial months in green and gold leaning on Ellis for almost everything, understanding the game and the position, along with getting through what Gallego called the "rough parts of the season."

"It's time for Penny to be in control," Gallego said. "Now, you can hear him barking things off to [Kevin] Kouzmanoff; you can see him discussing things with Ellis as opposed to just listening. Him and Mark are the leaders. He's the leader of the left side of the infield, and Mark's the leader of the right side."

"We talk about that a lot," Pennington said. "You have to be the quarterback of the infield. It's a big step I'm trying to take this year, being more vocal and not just knowing my role but everybody else's. Obviously, Ellis helps me out a lot, but I'm trying to gain a comfort zone on my own out there."

Pennington admittedly still feels like "the new guy" in the A's clubhouse, but his increasing stature on the infield has many around him thinking that newness is wearing off quickly.

"It's going to be a shame if his efforts aren't recognized at the end of the year," starter Dallas Braden said. "You want to talk about a guy who just absolutely brings this club plays day in and day out, it's just becoming comical. It's almost unfair because I almost expect it out of him."

Pennington, meanwhile, expects it out of himself.

"Obviously, 18 errors is more than I want," he said, "but I try to look at it more of, how many have I taken away versus how many have I given up? You've got to make the routine plays. That's why you're here. But, at the same time, it's fun to take away a hit from somebody."

Fun isn't just had on the field for Pennington, though. For as much as he's matured on the diamond, he's grown up in a bigger sense off the field, where he and wife, Missy, welcomed their first child last November.

Brady, nearly 10 months old now, has eased the roller coaster feeling of the 162-game grind, the one that is now viewed through a completely different lens.

"Brady doesn't care if I made an error or struck out three times," Pennington said. "Obviously this is a job and I love playing, but being a dad puts everything in perspective and helps you relax because of the fact it is just a job."

"With the new addition, there are new responsibilities," said Gallego, who has three children of his own. "It's a new outlook on life. With a family, I think Penny's starting to realize a baseball player is not who he is. It's what he does. He's a husband and father first. It eases those bad nights at the ballpark when you have that smiling child waiting."

Gallego, who spent 12 years playing in the A's organization, exudes a unique commitment toward his players that makes it seem he knows them just as well as they know themselves. Thus, he's also not afraid to brag about some of the more modest ones, Pennington included.

"We've all known Cliff has the ability and the tools to be a Major League shortstop," Gallego said. "But he needed to mature and grasp a hold of the mental part of it -- the day-in, day-out grind of success and failure. The mental side of the game is always going to be the last thing these young players are going to control. But if they can do that, they're going to be successful. Cliff has done a good job of doing just that. He can be a hero one night and come back the next day and be the same Cliff Pennington who gets his work in."

That doesn't mean he can't add a little something to his pregame mix, though.

"There's nothing wrong with walking around with a little bit more confidence like he seems to be doing," Gallego said. "If you're going to walk with a big stick, you better be able to handle it and use it, and he's proving that he belongs here. Players think they're proving it to their peers, their manager, their general manger. For me, they're proving it to themselves. If they can prove it to themselves, all the rest is gravy.

"He's gotten to that point, and I think we're going to see Cliff Pennington at shortstop for a very long time. I think he's starting to make a name for himself. Teams that we play on a consistent basis in our division, they know who Cliff Pennington is. The rest will know soon as well."