At some point (possibly at the beginning), a person trying to become a better pianist asks a question of what to do to play the keyboard effectively.

Here are some points and examples to keep in mind:

There is a kind of "optimal" body position in relation to the keyboard which is unique to each person. Yet, there is a "good" position range and "bad" position range. These ranges had been "arrived at" through trial and error after centuries of people trying to learn to play the keyboard instruments. The general rule is, if you feel any sort of tension/discomfort sitting in front of the piano, without playing, you need to fix it. (I am not talking about the general discomfort of having to keep your body upright, without being allowed to lie on the floor). If you don't feel tension/discomfort, but start to feel it after playing something simple, you need to fix it. Maybe I should say "you ought to fix it", instead.

Common problems are, people sit too close to the keyboard, without allowing enough room for the motion of the elbows (elbows are blocked by the ribs).

Best exercises would be to play the same harmony across all registers (e.g., C and G, from bottom to top, crossing hands: LH, RH, LH, RH, LH etc). Press the pedal down and play. If you can do it comfortably, without strain and constant readjustment of your body (other than the necessary movement of the arms) you are probably ok.

Here are some examples of people sitting too close (watching just the first couple of seconds is enough):

In this second example, you get a good demonstration of how such "too close" position is achieved from standing next to the keyboard (memo to self: chairs can move back).
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*NB: Note that these people might sound "great", if you yourself are yet to play pieces of similar complexity.

And here are some examples of professionals - pay attention to their relative torso position (and the angle of their upper arms):
(Please note, that the functionality that automatically takes the video playback to the correct time is not working at the moment, so you might need to sehift the slider manually)
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(NB: the video below is 3x slower than real life)
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In the video below, notice that, due to the pianist's body size, his upper arms are sort of flared out (they might not extend forward as much, but they are not pressed towards the ribs):
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1b) Related to the torso position, is the general unwillingness of people to train their feet to be planted close to the pedals - so that they are ready to use them, when the time comes.
(again, observe the first  "beginner" video above, and compare with the first "professional" video. (memo to self: feet are never bent under the chair).

Another problem is slouching. And I am talking about the bad kind, where the person refuses to support the spine with the muscles of the back and the abdomen, not making it like a strong tree trunk.
Here is a good video which shows just that (notice that in terms of elbow room, there is no problem):
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There are two main issues:
1) tall people sit too high, often (for lack of appropriate seating devices, I guess).
2) a common tendency to sit "comfortably" in the chair, leaning into its back (note, proper piano benches do not have backs).

With piano, I believe, you are better off sitting on the chair's edge.
Yes, it makes your feet work harder, and forces your abdomen to pull in (but that's usually a good thing, right?).

Why do I think so?
1) My teacher's told me so.
2)When you press on the keyboard, there is a certain kickback force that your body has to counteract.
You do it by having firm points of support:
a) Middle of the body on the chair,
b) Feet on the ground.
The feet have superior control when the point of chair contact with the thigh is closer to the middle of the body.
Sitting "into the chair" moves the point of contact with the thigh closer to the knee.

In any case, observe some of the professionals:

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I would be lying if I told you that all professionals have perfect upright postures. It is not true (as you can observe for yourselves), and in a lot of cases their careers come to an early end because of that.
This general "leaning forward" with the head bent forward slightly is a common trait among many pianists. I don't condone it, but it's there.

The more important point is the positioning of arms and fingers at the keyboard, so that the actual pushing of the keys can occur most effectively.

This brings us to the second point: what do you do with the fingers to press down the key(s)?

FINGERS (what people are usually most concerned with).

Too many books have been written about this subject. If you want to make this into a serious study, you should read them. One good book I can recommend is by George Kochevitsky, The Art of Piano Playing. It is located online here. For the quick opinion, however, based on my experiences/biases, read on.

As with the torso position, there are certain "good" ranges of finger behaviors that are found across the vast variety of professional pianist population. Below, I will attempt to show and contrast some videos.

I don't see much value in the "imagine your hand over a round ball idea" that you see in so many books and hear from so many teachers.
No professional plays like that, so lets just set it aside and talk about what actually happens when people play.

The fact that we have (usually) 5 independent fingers gives us an ability, in theory, to bend them in rapid succession, playing the selected 5 keys over which the fingers are positioned.
The flexor tendons (those that run on the palm side of your hand/forearm) pull the chosen finger towards the key surface, eventually bringing it into contact with the key, like a mechanical lever, acting upon another lever.

After the initial point of contact of finger flesh with the key surface, a number of scenarios can unfold:

*The point of contact between the finger pad and the key remains the same - the key is moved down using the overall action of the various joints of the fingers and, most likely, the arm.
*The point of contact between the finger pad and the key slides towards the front edge of the key, if the finger-hinge (i.e., the knuckle) remains stable.

Observe it in the following video:

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While, there is a room for occasional "whole arm" playing - I also refer to it as "pecking", because it has some similarities to chicken pecking - technique, to develop solid technique you need to work on the "slide" technique more.

Efficiency comes from minimal motion resulting in maximum achievement, and in the "point of contact remains the same" case, you end up involving a lot of other parts of the arm, not just the finger itself. If you want to, ultimately, be in control of the piano, work on getting rid of "large arm appendage" being involved when all you are doing is pressing the key down (quietly).

Observe some close-ups of professionals again:
1st clip shows Vladimir Horowitz bang through some big piano stuff in slow motion. You can clearly see that there is no sliding going on here. He is playing big octaves loudly, having to engage the entire arm appendage in addition to the fingers:

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2nd clip is of the same video, starting a little later on. Here, also in slow motion, he is playing a different kind of passage and the fingers are clearly sliding.

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In the next video, you get two pianists. Notice what happens during the short snippet (which flies by quickly): at first, the guy (Chick Corea) plays some accented/punched notes, not too quickly, and the "pecking" is prevalent, and then music changes to a faster passage - and immediately the "sliding" begins. Then the camera switches to the girl (Hiromi) and you see some similar action:

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Considering that each individual finger is a kind of controlled mini lever, each finger needs to be given the best "chance" to be a good lever, by positioning it above the right key appropriately.

Slight step back. The natural position of the hand, if you bend slightly the elbow and lift the upper arm from the shoulder, while sitting at the piano, is "sideways"(with thumbs being above everything else).
So, that's the "default" we all have to begin with. (Do it, and relax the forearms - don't move them any).

To allow the lever-fingers to press the keys, we have to do a rather "unnatural" but necessary rotation of the forearm to bring the hand assembly with all of its fingers into a plane parallel to the piano keys (hovering slightly  above, with finger tips extended over the key surfaces).

What most people don't realize is that you often have to go even further.

The knuckles in the knuckle bridge, the "finger pivot hinges" at the top of the hand, are directly responsible for how the finger strikes the key.
The most efficient way to press the key with the finger is by keeping it in the same plane of motion: the key moves down, and the finger moves down.
Oftentimes, the 4th and 5th fingers move on a diagonal against the key motion, because the knuckle is thus positioned, in turn, because the forearm is not rotated enough.

Here is the video illustration:
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As you can see in the video, most (non-professional, and some "professional") pianists play with that slope towards 4/5. You really should want to not do that to play better, and yes, it means fingers 4 and 5 work harder - but that's life, and it gets you results.

The bottom line is, there are some bad habits you can get into, and there are some choices you have to make.

Be professional by doing everything as well as you can in the limited time that you allocate for yourself.

If you have made it this far, you might want to read more on other Piano Technique pages or go back to the main NCSU page.

Video Embed Test below: