小节是音乐作品中最基本的有规律的节奏单位，表示乐曲强弱起伏的基本结构。每个小节内含有和拍号标明的拍子数相同的节拍。一个小节通常以强拍开始，称为完全小节。所有小节重复前面小节的节拍，如三拍子，一个小节内会有强-弱-弱三拍，四拍子一个小节内有强-弱-次强-弱四拍，其他小节反复进行。

以弱拍开头的小节，叫做“弱起小节”。弱起小节开头的乐曲有两种，一种是直接以完全小节结尾，一种是在最后一个小节中加上一个强拍，使其和弱起小节的拍子总数合在一起成为一个完整的小节。这种结尾的小节称为“不完全小节”。。

乐谱中以纵线分割各个小节，这个纵线叫“小节线”。依不同的指示用途，小节线分为单小节线，双小节线小节线是一条垂直线，通常画在第一拍前。双小节线则有两种：用来分隔乐曲各部分时，会画成两条等粗的细线；作为乐曲的结束的称为终止线，是内细外粗的双直线。当乐曲需要重复时，重复的部分也会用内细外粗的双直线表示，并且在细线旁加上垂直的两点。

In musical notation, a bar (or measure) is a segment of time corresponding to a specific number of beats in which each beat is represented by a particular note value and the boundaries of the bar are indicated by vertical bar lines. Dividing music into bars provides regular reference points to pinpoint locations within a piece of music. It also makes written music easier to follow, since each bar of staff symbols can be read and played as a batch. Typically, a piece consists of several bars of the same length, and in modern musical notation the number of beats in each bar is specified at the beginning of the score by the top number of a time signature (such as 3
4), while the bottom number indicates the note value of the beat (the beat has a quarter note value in the 3
4 example).