The measure, or beat, is the set of values on the staff definied to the earpieces, or rather vertical lines that divides in various spaces the staff. The last measure of a composition is always closed from a double earpiece.
Inside a measure there are musical values, notes, whose addicion should give a constant value. This value is estabilished at the beginning of the song, though a fraction in which on the denominator we’ll have a time unit (4 is a seminim), while at the numerator the amount of time units presents. In a beat of 2/4 we’ll have at maximum two semiminimum notes (=1/4). The following example will clarify the ideas.
Beats are divided in simple measures, those in which every time is represented to a easy value, or rather when at the numerator we see the numers 2, 3 e 4. They’re classified in binary, ternary e quaternary’s measures.
We know compound measure those in which times can be divided in three. If you want to obtain the corresponding measure, composed by a simple that should be multiplied for three the numerator and for two the denominator. Es: 2/4 = 6/8 ; 3/4 = 9/8 ; 4/4 = 12/8.
We’ll not stay here saying too much about this topic because we’ll see, meticolously how we should behave, by the metric/stylistic point of view. This is not a deeply topic, that i understand can be difficult, but essential to start.
Let me conclude mentioning the metronome. This is the instrument though is possible measure the time and the rhythmic scan. Like a pendulum, it moves his “pointer” to the right and left, everytime with a touch. Every touch corresponds (ex. in a measure of 2/4) a semiminim note (a quarter) that should durate until other touch’s arrival. Two completes touches, in this mode, will complete a beat.
We’ll see this concepts on firsts practise’s exercises.