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As i know the only thing that top number of time signature defines is beats in a bar.

But i can read a music sheet without bar lines. So only the bottom number of time signature is required.

Why top number of time signature/dividing music into bars is required?

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You can (theoretically) read a score without bar lines, and indeed bars (or measures) and bar lines, in the sense that we use today, are a relatively new invention, from the mid of 17th century. Before that, "bar" lines were not used at all, or were used only to visually divide a piece into sections or phrases. In fact if we go all the way back to Gregorian chant, even the concept of beat, or regular time counting, is somewhat vague or even non existing.

But as European erudite music evolves, a number of things happen:

  1. Stylistically, the beat or pulse of the music becomes more important and a distinctive integral part of each particular musical style. The rhythmic division (binary and ternary) and the existence strong and weak beats, important in popular music such as dances and work songs, is also incorporated in erudite music.
  2. Musical composition becomes more complex, with multiple instruments/singers performing different parts and polyphony (two or more simultaneous lines of independent melodic parts) becomes more and more important.

And so it became necessary to have a notation device that allowed for the correct understanding by the performers of the intended rhythm and synchronization of the different performers.

Today the concepts of measure and time signature and the bar notation are fundamental parts of western music, that makes possible in a very practical way, for example:

  • To identify a binary meter, like a march, or ternary meter, like a waltz.
  • To identify at a glance the strong and weak beats of each measure.
  • To notate extremely complex rhythmic figures (including polyphonically), with irregular subdivisions, while at the same time maintaining a clear perception of the beat.
  • To precisely notate pieces with complex signature (5/4, 7/8, etc.).
  • For an orchestra or ensemble to rehearse (measure numbering is a fundamental part of instrument-parts scores).
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Here's a little test of your ability to read music without a time signature. This is the theme from a piece by a very well known composer - note, I transposed it into a different key because the original key might frighten some people. Which is the correct version - 3/4 time or 4/4?

(I'll add a link to the original after a day or so, to give the OP time to answer the question)

Public domain music - image created by me

Apparently the OP didn't bite, but it's not a (rather insipid) little waltz or minor-key ballad in 3/4 time, which is what the first version would suggest.

The 4/4 version is actually the subject of a Bach fugue (BWV 853): see page 3 of https://www.pianoshelf.com/sheetmusic/735/bach-bwv-853---wtc,-book-1--prelude-and-fugue-no.-8-735

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Most (Western) music divides neatly into regular sections. We call those bars. Without them, there would be no particular emphasis on the first beat of each bar, which is where the composer got the top number from. When he wrote the music, he felt it had a rhythm, and translated that into number of beats - the top number. The bottom number, as you state, tells what sort of beats those are.

A vague analogy is writing some sentences without punctuation or a gap between each word. Yes, it could probably be read, but where's the advantage?

If you were presnted with a piece of music written with no barlines, how would you know whether it was, say, 3/4, 4/4 or 5/4? There would be few clues, and the feel between even those three are very different, so you wouldn't be playing it in the intended way.

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Why, in that case, is the bottom number required either? Use time signatures and bars if they help organize the rhythmic patterns of your music. Most composers find they do, both for musical and practical reasons - what would you prefer, "go from bar 123" or "go from the 1,103rd note"? :-)

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