Among everything printed on a page of music, the small numbers over the notes are the thing adults teaching themselves ignore most consistently. They look like an afterthought. Nobody explains them. The notes are obviously the important part, and the numbers seem like a fussy detail for people who care about that sort of thing.
I want to make the case that they are not a detail, and that ignoring them is one of the most expensive habits available to a beginner — expensive in the specific sense that it costs you weeks later, invisibly, in pieces that will not come together and you cannot say why.
What the number means
Every finger has a number, on both hands, and it is the same on both: thumb 1, index 2, middle 3, ring 4, little finger 5. Not the right hand counting one way and the left the other. Both thumbs are 1.
A number above a note is an instruction about which finger plays it. That is all it is, and it is why it looks unimportant.
Why it is not a suggestion
Here is the thing that is rarely said out loud: a fingering is a plan for the whole phrase, not a preference about one note.
The hand has five fingers and the keyboard has eighty-eight keys, so playing anything longer than five notes requires the hand to move — and the move has to happen somewhere. A printed fingering is somebody's solution to where. It puts the thumb in the place where passing under is easy, it arranges for the long fourth and fifth fingers to avoid the awkward keys, and above all it arrives at the end of the phrase with the hand in a sensible shape for whatever comes next.
Choose your own fingering note by note, as most self-teaching adults do, and each individual choice is perfectly reasonable. It is the accumulation that fails. You get four notes into a run, find your little finger on a key with three more notes to go, and stop — not because the passage is hard, but because you have run out of hand. Then you invent a jump to recover, and the jump becomes the thing you practice, and the piece has a permanent stumble in it whose cause is three notes earlier and entirely invisible.
Adults describe this as there is a hard bit in the middle I cannot get past. Very often there is no hard bit. There is a bad plan.
The second reason, which matters more than the first
A fingering you keep is a fingering you learn.
Your hands remember movement. If you play a passage with a different fingering each time — and you will, if you have not chosen one — your hand is being given a new problem at every repetition and nothing consolidates. You can play something two hundred times and it stays effortful, and the effort feels like evidence that you are not talented.
Playing it the same way forty times is worth more than playing it two hundred different ways. That is most of what fluency is: not speed, but a movement your hand no longer has to think about. Fingering is what makes the movement repeatable.
What to do, practically
Use the printed fingering first. Not out of obedience — out of economy. Somebody has already solved the puzzle and you get the answer for free. Try it slowly before deciding it does not suit your hand; a fingering that feels wrong at first very often turns out to be solving a problem three bars ahead that you have not met yet.
If the printed fingering genuinely does not work for your hand, change it — and then write it in and never change it again. Hands differ. Some are small, some have a short little finger, some cannot comfortably stretch an octave, and a printed fingering is not a moral law. Changing it is legitimate. Changing it repeatedly is what does the damage. Pencil, not pen, and commit.
Where there is no printed fingering, decide before you practice. Play the phrase slowly, find where the hand runs out, put the thumb there, work outward in both directions, and write it in. Five minutes of this before you start learning a passage will save you an hour of grinding at it.
Practice the thumb-under move on its own. The single most common place a plan falls apart is the moment the thumb passes beneath the hand. Done well it is silent and invisible: the thumb travels early, under a hand that stays quiet and level, and the note it lands on does not bump. Done badly the wrist jerks sideways and every scale has a lurch in it. Isolate three notes around the crossing and repeat those three notes alone, slowly, until the bump is gone.
Say the numbers out loud while you play slowly. It feels absurd and it is extremely effective. Saying one two three, one two three four fixes the plan in a way that silent repetition does not, and it catches the moment you have quietly used a different finger.
One thing not to do
Do not write the finger number above every note. Write it where the hand moves — where the thumb passes under, where the hand shifts position, where a phrase begins. Numbers on every note become the thing your eye reads, exactly like note letters, and you stop reading the music. A handful of numbers in the right places is a map. A number on everything is a crutch that replaces the map.
Why this matters more for you than for someone with a teacher
A teacher's hand comes down on the keyboard and says no — third finger there, or you will be stuck in two bars. The correction takes two seconds and it happens before the bad habit exists.
Learning alone, nobody says it. The wrong plan is not painful and it does not sound wrong, so there is no signal. It simply means that everything is slightly harder than it should be, forever, and you have no way of knowing that it is not supposed to be this hard.
That is the real argument for taking the little numbers seriously. They are the closest thing a printed page has to a teacher's hand.
The part you cannot see
Whether your thumb is traveling early or arriving late, whether your wrist jerks at the crossing, whether you are using the same fingering twice in a row — none of these are audible to the person playing. They are perfectly visible from the side.
So put your phone at the side of the keyboard, at about the height of the keys, play a scale or a passage with a position change, and send it. The written feedback is free, and it will name the specific place where the plan breaks.