From then on, Sherlock learns a new language. It is the language of John's facial expressions – funny, how he'd always thought of them as clues to a case or otherwise generally unreadable, but on John they spoke entire novels – and of body language. John began to exercise control over Sherlock in infinitesimal ways, just to see his eyes widen with unusually slow understanding.

They are standing two feet apart in the living room, surveying each other with a kind of coy, yet gruff, pride as they dressed for Mycroft's dreaded birthday meal. John hadn't asked, but he suspected it was his fortieth, the amount of fuss that Mother Holmes had been making, and so they were dressed appropriately – John in a dark grey suit, Sherlock in black with his purple shirt beneath. Sherlock is fully dressed, unable to dawdle as always, yet John has only his suit trousers on, his shirt unbuttoned.

"Fasten my buttons." he says, his eyes looking steadily up at Sherlock. Weeks ago – was it only weeks? Sherlock wonders – he would have scoffed, tossed some snide remark about John's ability to shoot a man through a pane of glass dead on and wasn't he capable of fastening his own clothes for God's sake, but now he hesitates for only a moment. The tone of John's voice, the unwavering , expectant gaze – this is non-negotiable. And so, he starts at John's stomach, which barely flinches at the chill of Sherlock's long fingers working deftly around the black fabric. Moving up to the chest, taking in with unabashed admirating the light scatter of hair, Sherlock feels like mouth turning to sandpaper and an inability to look John square in the eye. John, of course, notices this.

"Look at me while you're doing it."

His tone is not angry, nor forceful. He knows Sherlock will obey, even if Sherlock doesn't know why. And so he lifts his eyes almost warily, his fingers missing the next buttonhole without his eyes to guide him – his knuckle skims John's skin, which seems very warm against his own. John stiffens for a moment but says nothing. He continues to button, slowly and methodically, until he reaches John's neck, the curve of it reminding Sherlock of pieces of music he thought he'd forgotten. They are face to face, Sherlock standing far closer than is necessary given the length of his arms, and John is still looking at him with that serious, steady face. He leans towards John, millimetres at most, and instantly John places a rigid palm on Sherlock's shoulder, preventing him from going further. His gaze is no longer merely serious – it is one of disapproval.

"Greedy." he chastises, and pushes Sherlock away, fastening his own top button. Sherlock swallows, feeling as though he has been robbed of something exquisite, aching to finish this oddly entrancing task.

"You'll learn," John says, the conversational tone returning, his face now turned to the mirror as he loops his tie around his neck. Sherlock thinks he is beginning to understand the rules.

The meal drags, John behaving – disappointingly, in Sherlock's own befuddled opinion – entirely normally. He is charming with Sherlock's mother, endures Mycroft's accurate yet inappropriate jibes about their blossoming – well, whatever it is, and smiles at Sherlock once over the cheeseboard with what looks like unadulterated happiness and warms Sherlock's ears. He has an easy charm about him, Sherlock notices, with everyone from waitresses to Sherlock's peevish aunts, and yet he beams at nobody like he does Sherlock. The thought makes Sherlock feel slightly seasick.

Back at 221B, Sherlock flinging his suit jacket and shoes into a corner of the room unlit by the television, John pours two glasses of brandy and gestures at the sofa. Flicking a longing glance at his armchair, Sherlock sits awkwardly beside John, who lets his knees fall apart and his head back onto the cushion, utterly relaxed.

"You can unbutton my shirt now," he says, and Sherlock feels a now-familiar tightening in his stomach, sickening lust at the orders of this sandy-haired mystery. Yet the tone is not quite as authoritarian as earlier, and so although he begins as he was told, reversing his earlier path and starting at John's taut neck flung over the sofa's edge, he speaks softly.

"What is this, John?" It pains him to ask, and it tells in his voice, tighter than normal. One button comes apart. John shifts slightly but doesn't lift his head.

"You're unbuttoning my shirt. Have you lost your deductive powers?" he replies, but the tease has an undercurrent of confusion. It is an alarmingly vague question from the man who demands precision.

Another button, slower. "We are – well, sleeping together. Sometimes. And I told you I loved you. And now you're ... ordering me around. Which is - good, but..." These words he says as stiffly as he knows how, determined not to show weakness. There are some rules he hasn't learned yet. John opens his eyes, although still doesn't move.

"Yes." Another button, and a thumb resting lightly on John's chest for a second, unwilling to continue before he knows where he is heading. "And here we are, you undressing me on the sofa," John adds, lightly. Sherlock withdraws his hands.

"Don't make light of this, John," and something hurt in his tone finally brings the doctor's head back upright. Sherlock's voice has never sounded like this, needing any reassurance.

"I'm – not. I don't know what this is either." John says warily. Sherlock's face falls slightly and John feels immensely responsible. "I mean," he tries again, "- do we have to put a name to this? You know how I feel about you -" I adore you, I could fuck you every morning until forever, you made me remember how to study every detail of a persons' face and commit it to memory, John thinks.

This appears to be entirely the wrong thing to say.

Sherlock stands and paces for a few seconds before seating himself in his armchair, several stretched feet of miscommunication away, his hands steepled to his forehead.

"Do you know why I don't have relationships, John?"

"No." He has never asked. Assumed there would be no response. It is only since he realised with a jolt of absurd longing that he is accidentally in love with Sherlock that John has given much thought to his relationships, or lack thereof.

"There was somebody, once." Sherlock says in a tone that knocks John's stomach, "He was – I became obsessed. Like I am with my work, he was everything. And then he left. It is not an experiment I wish to repeat."

Rational thought leaves John for a second as he imagines his fist connecting with the cheekbone of a man who could leave Sherlock Holmes, and he is filled with a desperate need to comfort that he knows will be rebuffed. He then thinks, panicked, that he is being bestowed with an enormous responsibility. He decodes Sherlock's statement.

"You don't want to be left?"

"I couldn't. No drug could numb you leaving. I am not trying to be melodramatic, John. But I mean that if this is – a phase," he spits this phrase and John feels bile in his throat, the word sounding parroted from the mouth of a bigoted acquaintance from Sherlock's past, "- or you are merely trying to get over a brief fascination with me, then I must ask you to stop it now."

John finds himself choked up, absurdly and without warning. He is torn deeply between everything he knew, life before Sherlock, and his life now – boundaries crosses, sexualities entirely ignored, the absolute ache in his stomach as he wakes to the sight of Sherlock's hipbones and stomach and loose limbs, the giddying thrill of lowering his voice and watching Sherlock's eyes blow wide as he obeys John's bidding. He knows the next few words could make all the difference and as he takes a deep breath, prepares to make a promise he's almost sure he can keep, Sherlock turns his face towards John with eyes wider than saucers, open and pleading. It wrenches John open and leaves only one response.

"Yes," John says, automatically, in response to nothing and everything, getting to his feet and crossing the carpet. He kneels beside the chair and holds Sherlock's stare, urgently.

"Yes, I will be here. I won't leave you, Sherlock." He is sure now; the final piece of this puzzle falling into place. Sherlock needing him, looking with pleading eyes – it causes an ache in his throat, but it soothes the nagging fear at the back of his mind, the one he had discussed with his therapist so many times – "Sherlock doesn't need anybody. He doesn't let himself,".

At John's words, Sherlock's shoulders loosen and he crawls, like a child, onto the floor, curled round John's knees, and breathes, "You make it quiet inside my head."