Jack Wilshere refers to it as "the good one" and he has done so, on a couple of occasions of late, in order to offer reassurance. "Don't worry," the Arsenal midfielder has said. "It's the good one."
The good one is the left one; the left foot; his favoured foot; the one that has not borne the brunt of all the injury frustration. That would be the right one; the bad one; the one that took away 17 months of his young career and retains the capacity to worry just about everybody connected to Arsenal and England.
It is impossible to consider Wilshere, particularly in World Cup year when everything is ratcheted up that little bit further, without discussing the state of his ankles. When he takes a knock in training or goes down in matches, everyone is thinking the same thing: please, don't let it be bad. Or, please let it be the good one.
The good one has sparked a few scares, most notably at the end of last season when, having overcompensated to protect the right, Wilshere felt it buckle. It put him out for six weeks and he was subsequently able to start only two matches in the final two and a half months. He has taken further bruises to it in recent weeks. But, as he has said, it has been nothing to worry about.
And here is the thing, the really encouraging thing for both club and country. The general fretfulness about Wilshere has been overtaken by excitement, the kind that he inspired during his breakthrough season in 2010-11 and has done, in flashes, since his comeback in October 2012 from the lengthy lay-off. Against Bayern Munich last February, for example, in the Champions League last 16 home leg, he stood tall as others shrunk.
Wilshere has craved a consistent level of performance, rather than mere big-game high spots, and the signs have come to look increasingly positive. He has been the man of the match, or pretty close to it, in Arsenal's past four fixtures – the wins over Newcastle United, Cardiff City, Tottenham Hotspur (in the FA Cup) and Aston Villa.
At the beginning of the season, there had been dark mutterings about Wilshere's game. He had undergone an operation at the end of the previous campaign to remove a pin from his right ankle that had caused discomfort and, although he enjoyed a pre-season for the first time since 2010, he did not look comfortable. He waded rather than glided. Arsène Wenger admitted that he was not fully fit.
The handbrake, to borrow the manager's phrase, is now off. When Wilshere has pressed the accelerator, he has found the response of old and it has been heart-warming to see him zip over the first few yards, past or away from opponents, chest pushed out, with echoes of Paul Gascoigne.
Physical fitness, and confidence in said fitness, is everything and it has underpinned Wilshere's purple patch since he returned from the two-game ban that he received for raising his middle finger at Manchester City fans on 14 December. He missed the derbies against Chelsea and West Ham United but, at least, it spared him the complete Christmas and New Year shake-down.
Wilshere knows that no player can last the entire season, particularly one with a chequered injury history, and he has learned to manage his body. So, too, has Wenger, who is on record as saying that he was guilty of over-playing Wilshere in 2010-11 and 2012-13.
Wenger has rotated him at times this season, although that has been easier to do after the summer signings of Mesut Özil and Mathieu Flamini. The Arsenal midfield boasts formidable strength in depth and, when Wenger makes changes to the department, there is little or no shortfall in quality. The competition has further fired them. Wilshere's form, for instance, has made light of the absence of Aaron Ramsey and the Welshman has been the club's player of the season so far.
Ramsey injured a hamstring at West Ham on Boxing Day yet he is close to a return and it could pose a problem to Wenger. Wilshere has thrived in the past four matches in a central position, either as the more advanced of the two midfielders in front of the back four or in the No10 role.
He has made no secret of his desire to be given a forward-thinking, central brief, rather than be accommodated off the flank and it is clear that his impact is lessened when he plays as a wide midfielder. His game is built on the ability to slide through-balls, to unpick defences or drive directly at them. Then again, so is Ramsey's and he, too, prefers a creative role in the middle. Arsenal, meanwhile, look better balanced with a defensive midfielder and Özil, the principal No10, has to play.
Wilshere's goals have been another boost, he now has five for the season with the touch and finish at Villa Park on Monday night. His tally across his previous seasons at Arsenal had stood at five. The 22-year-old is aware that he needs to contribute more in front of goal and it was interesting to hear him name-check his England team-mates Frank Lampard and Steven Gerrard after the Villa game as examples of where he wanted to be.
"I'm not really a defensive midfielder … I'd say I was an attacking midfielder and if you are going to be one of the best, you have to score goals and create them," Wilshere said. "If you look at the best midfielders, your Lampards and Gerrards, they've done that."
Wilshere remains an international rookie, having won only 13 caps and yet the burden on his shoulders at the World Cup stands to be onerous. It is partly because the England team is hardly of vintage quality but also because a fully firing Wilshere is a tantalising prospect. Continue in this vein and hopes would swell. The good one and the bad one are on the right track.