In music theory, a time signature (also called a meter signature) is a notation used at the beginning of a piece of music to define its rhythmic structure. It indicates how many beats are in each measure (bar) and which note value represents one single beat.

1. Structure of a Time Signature

A time signature typically consists of two numbers stacked vertically:

1. Top Number: Specifies the number of beats contained in each measure.
2. Bottom Number: Specifies the type of note that receives one beat.

2 = Half note

4 = Quarter note

8 = Eighth note

16 = Sixteenth note

1. Types of Time Signatures

Time signatures are classified into three primary categories based on how the beats are grouped and subdivided:

1. Simple Time: Each beat can be divided into two equal parts (e.g., one quarter note divides into two eighth notes).
2. 4/4 (Common Time): Four beats per measure, quarter note gets the beat.
3. 3/4 (Waltz Time): Three beats per measure, quarter note gets the beat.
4. Compound Time: Each beat is a dotted note that is naturally divided into three equal parts.
5. 6/8: Two main beats per measure (dotted quarter notes), each containing three eighth notes.
6. 9/8 or 12/8: Three or four main beats respectively, with eighth notes grouped in threes.
7. Complex (Odd) Time: These signatures have an irregular number of beats that cannot be divided into equal groups.
8. 5/4 or 7/8: Often used to create unique rhythmic feels, such as in the "Mission Impossible" theme (5/4).
9. Common Symbols

Some time signatures use symbols instead of numbers:

1. C (Common Time): Equivalent to 4/4 time.
2. Cut Time / Alla Breve: Equivalent to 2/2 time, which indicates two half-note beats per bar and is typically played at a faster tempo.