Sherlock strode back through the frigid air toward 221B, frustrated. He'd expected to be travelling out to the Holmes family estate by now. Mycroft, for once making a New Year's resolution he could keep, had finally made certain legal arrangements that needed signatures, witnessed by Mother and her solicitor. That was irritating bother enough. But then he'd left his wallet at Baker Street, and so missed his train.

Mrs. Hudson had propped the front door open, apparently while she went in and out with a good deal of post-Christmas shopping of some sort, to judge from the bags still in the cab parked by the curb. Sherlock slipped in and ran lightly up the stairs. He'd grab his wallet and be back at the station before the next train to his destination left.

But as he neared the door to the flat, he instinctively stopped at the unfamiliar stimulus. It took him a moment to identify it, and another moment to confirm that its source was indeed within the flat. It was music, played on an acoustic guitar. Or it would have been, he thought, if the player's amateurish strumming, halting for agonizing moments between chord changes, could be said to be 'music'. Worse, the noise was being accompanied by someone half-murmuring insipid lyrics.

Acerbic comments automatically rose in Sherlock's throat as he reached for the door handle. He could just picture what he would find – John feigning polite interest as his latest female acquaintance regaled him with her so-called talent. It would be doing John and the world at large a favor to interrupt, and to shut the dreadful dissonance down with a cutting remark.

But his hand paused a fraction of an inch above the door handle, a fraction of a second before contacting it. Finally his brain had finished processing the input of the vocal sounds. No doubt the extra time was due to a double check for confirmation – the voice was John's. Sherlock had never heard him sing before, and, intrigued, he paused to listen.

Oblivious to Sherlock's presence on the other side of the flat door, John continued to struggle through the piece. Sherlock gathered from the lyrics that it was called "Four Strong Winds", and that John knew how it was meant to sound. Now that he was paying proper attention, Sherlock grudgingly admitted to himself that John's surprisingly smooth tenor carried the tune pleasantly enough, but that, obviously, coaxing his fingers to produce the required chords from the guitar was proving to be a challenge for him.

Sherlock once again reached for the door handle. He hadn't even known John owned a guitar; perhaps it was a recent acquisition. Some people, he mused, ought not to have access to musical instruments. There was quite enough unlistenable dreck in the world already, without over-aged novices adding their lamentable contributions.

And yet … John's voice carried with it a surprising sense of contentment that gave Sherlock pause. He suddenly recognized that this was a private moment. To enter now would be … well, a 'bit not good', as John himself might say. Although still very strongly tempted, Sherlock deliberately drew his hand away and stepped back.

Feeling smug for having made the correct observations and conclusions in good time, Sherlock retreated silently back down the stairs. As he went, on a whim, he set himself an amusing challenge: to refrain from interfering with John's dalliance with music, as long as it lasted. It would be an interesting test of resolve, and certainly an exercise in self-control if he exposed himself to much more of what John probably thought of as 'practicing'.

Somewhat self-satisfied, he paused on the streetcorner to call John from his mobile, to tell him to bring his wallet to the station. With a last thoughtful look at 221B, he turned and strode quickly so as to arrive there before him.

Some months earlier, Sherlock had performed a routine hacking into John's online accounts. He had added himself to the share list for his flatmate's personal calendar, naming the supposed recipient "System Archive" in case John ever reviewed his account settings. Sherlock casually monitored the calendar's entries as a matter of course, and he did so now from the Holmes family home.

Looking at the period for which he was expected to be absent, Sherlock rolled his eyes at the number of dates John had set up, and snickered at the notes he'd attached to each to help him remember salient details about each woman and her life. Much more interesting, however, was a new appointment, labeled: G lesson 1500. The note was set to repeat indefinitely on the same day of the week. Sherlock doubted that it was coincidence that G lesson 1500 would be taking place at the same time as Lestrade's weekly staff meeting, held in the press conference room at the Yard. Sherlock regularly attended this, albeit surreptitiously. Each Wednesday midafternoon, he let himself in to a forgotten basement office and listened in by means of an old, dusty, and seemingly defunct internal secure conference call device sitting innocently on a disused shelf in the conference room, an unplugged electrical cord left dangling decoratively over the shelf edge to complete the illusion.

Hmm, Sherlock thought. It seems the period of my mental exercise may be extended.

Back at 221B, Sherlock diligently kept to his Wednesday schedule, leaving the flat each week with some comment or other to John about what nonsense Donovan was likely to spout this week, or whether Anderson would again blame another staffer for one of his own errors. Then, if he should instead circle back round and enter a nearby building from the back way, he might gaze down from an upper hallway window a few minutes later to see John emerge from the street door, case in hand, and walk cheerfully down toward the music teacher's studio three streets away.

Or, alternatively, if on any given day he were to rise several hours earlier than was his habit, and pad quietly down the basement stairs to listen outside the door to 221C, he would be certain to faintly hear the evidence of John's plodding improvement on the instrument through the solid door. Wincing at the results of every incorrect finger placement, or pursing his lips at the use of a tonic chord when clearly a seventh was indicated, more than once Sherlock had to firmly discipline himself to keep quiet until he could return to his room to mutter his reviews of the musical selection being worked on, the apparent deficiencies of John's teacher, John's sense of pitch, or whatever seemed to be most lacking at the moment. As the weeks dragged on, Sherlock couldn't decide if it were becoming easier, or more difficult to keep his knowledge of John's attempts at progress with the guitar from his flatmate. He yearned to give his opinions on the topic, occasionally telling himself that by speaking them aloud, he might be able to assist John in making more rapid progress.

But then, in less rationalizing moments, even he had to admit that John's dogged perseverance was clearly paying dividends, and also that should Sherlock breathe one word about it, he would likely stop immediately, refusing in embarrassment to trespass further on the territory already so competently occupied by Sherlock and his violin.

In this way, a year passed. There was a small difficulty midsummer when Lestrade changed the staff meetings to Thursdays, and John found to his dismay that the music teacher did not have a corresponding opening, but Sherlock soon adopted the habit of taking a turn around Regent's Park and Primrose Hill each Wednesday, "to think", as he explained to John. With a quick check of John's calendar, he noted that the lessons resumed unabated after only one having been missed.

A longer hiatus ended in the fall, as John stared uncomprehendingly at the utilities bill. "Sherlock," he said bemusedly. "It looks like we now have a rather large credit on the account. I doubt we'll owe anything for the rest of the year."

Sherlock didn't look up from where he was tapping out succinct replies to his email correspondents. "Billing error," he said crisply. "They've been overcharging us. I've had it corrected." He suppressed a smile and carefully appeared not to notice as John absently tapped his checkbook, his gaze having drifted in the direction of the music studio.

As the holidays approached once again, John's assigned repertoire turned to traditional Christmas selections. By now working on more complex plucking, strumming, and rhythmic techniques, John's fingers were beginning to exhibit a surprising ability to draw forth the essential joy in the familiar tunes, if not always to execute them perfectly.

Shivering in the unheated hallway outside 221C in the early morning hours of Christmas Day, Sherlock wavered, considering whether to reveal his now-regular presence there. Listening, his heart ached and his fingers twitched as, in his mind, he accompanied John's sung melodies and supporting chords with the violin harmonies he'd learned as a child for each of the carols, and still played privately every year with only recordings to carry the other parts. Sherlock had never been much of a man for spirituality, but there was something about the season's music that had always spoken to him. For a few brief days each year, a handful of favorite pieces turned his attention away from the constant flow of the gruesome results of humanity's baser instincts, and toward an uplifting appreciation that it was, on occasion, capable of achieving a much larger and more worthy perspective. John's voice, floating softly over the guitar strings' warm sounds, now resonated in genuine, if solitary, celebration of this. From the other side of the basement flat door, Sherlock observed that John's interpretation of the music's meaning paralleled his own, and reflected that the insights might be best savored if shared. Yet the detective's determination to prove his strength of purpose, if only to himself, caused him to continue to remain stoically silent.

It was the second week in January, and Sherlock remained undecided. Should he continue his weekly planned absences and his dawn vigils outside 221C? Or, had his year's success in keeping the knowledge of John's musical efforts to himself sufficiently fulfilled his personal vow? Each time he heard John's now-nimble fingers dance on the strings, he revisited the choice, but as yet he had not reversed it.

Now, huddled on the floor of the dark hallway next to 221C's door, knees drawn up and wrapped comfortably in the duvet he'd brought down from his bed, Sherlock sighed, having once again decided against revealing himself this particular morning. He nodded approvingly as John mastered an interesting chord sequence after only three runs. Soon his flatmate, a creature of extreme habit in matters left to himself, would conclude his allotted hour of practice and emerge. By that time, Sherlock would need to have returned to his bedroom and resettled himself for a couple more hours' rest before the day began. He began to gather the covers together and to pull his legs under him, ready to stand up and slip away.

Suddenly, 221C's door unlatched, and was drawn open. Sherlock's heart leapt to his throat, and he froze, blinking, caught crouching in the harsh light streaming through the doorway from the single bare bulb in the shabby flat's only fixture. Mrs. Hudson stood silhouetted before it, her hand on the door handle.

"Oh, I'm sorry, dear, I didn't mean to startle you. I hoped you'd still be here," she fluttered, opening the door still further to invite him in. "You nearly always are." She turned to John, who was staring open-mouthed, looking back and forth from her to Sherlock.

Startled and shocked, John stood too quickly, which only toppled the guitar dangerously towards the floor. He grabbed at it frantically, juggled it for a moment, and finally, gaining a tight, two-handed grasp about its neck, he jerked his head anxiously towards Sherlock. "Nearly … always?" he choked.

Then John looked aggrievedly at Mrs. Hudson. "First, you come in here this morning to tell me that you know I've been down here practicing, and now …" His voice sounded strangled.

"I'm so sorry, boys," Mrs. Hudson said. Her words and tone were apologetic, but to the observant, her eyes betrayed amusement. "I only found out about John playing down here last month, and about Sherlock listening in a week later. I made myself a very serious promise to never say a word about it to either of you, but, well, I've never been good at keeping my New Year's resolutions for very long."

John's mad scramble for his guitar and Mrs. Hudson's tittering had given Sherlock a few moments to contemplate. He gathered his composure and walked slowly into the room. His eyes flicked gratefully to the electric heater on the floor, warming the small space. He let the duvet fall from his shoulders into an untidy pile by the door, and he knelt to place the case containing his most prized possession carefully on the floor.

Diffidently adjusting his dressing gown, Sherlock removed his cherished instrument and checked the tuning. Standing once more, he pivoted to face John. "'Westminster Carol'", he said simply. "Please."

Several heartbeats passed, which felt more like an eternity. John hesitated, indecisive. But in hearing that one word, its sincerity proven by the presence of the violin in Sherlock's hands, John was flooded with all the praise he never expected for his hard-earned progress, approval freely given by the one person who opinion mattered most. At last, he smiled nervously, resumed his seat, and swung the guitar into playing position. Sherlock raised his bow, already looking forward to the new year.