The music is always present even ripple. We may not hear it clearly, but always feel. Each step of this ripple is called a fraction. The proportion varies by its power. There is a strong share, and weak.
A segment of a musical work from one strong share to another is called tact. Thus, the number of beats per measure is different depending on its size. Thus, the size of Vals - three quarters. This means that a shock it is, each of the first three quarter of the shares. In each step of the waltz will be exactly three quarters. And polka, for example, dicotyledonous dance. And in each step polka contains two quarters.
The size of the quantum is represented by a number, in which the top number indicates the number and the lower the duration of the shares that make up the beat. For example, 2/4, 3/8, etc.
The size of the quantum is set at the beginning of the work.
The bars may be not only simple, with the same emphasis, but also complex. Compound measures, size 6/8, 1/9, 4/4 and have several other similar stress one important thing, the most powerful, on the first beat of the measure; the other or others, if they are more on the relative strengths shares inside tact.
Cycles are symmetric (in 2/4, 6/8, etc.,) and unbalanced, not only in 3/4 or 3/8, but such as 5/4, 5/8 and others.
There are quite rare, unique sizes tact. So, in the finale of the Opera "the snow maiden" by Rimsky-Korsakov wrote the chorus of Slovenia God yarily the Sun to the size of a bar of 11/4.