Measure (bar)
A measure (in British terms a bar) is a group of beats from one strong beat to the next. In written music, measures are separated by bar lines — vertical lines across the staff.
Beats fall into groups of four, three or two, and the first beat of each group is the strong one. One such group is a measure. The time signature at the start of the piece tells how many beats it holds: four in 4/4, three in 3/4.
The bar line always comes right before beat one, the strong beat. It makes no sound and changes nothing — it only shows where a new measure begins. At the very end of a piece stands the final bar line, a double bar line made of a thin line and a thick one.
The notes and rests in a measure must add up to exactly the top number of the time signature, no more and no less. A measure of 4/4 is filled by four quarter notes, two half notes, one whole note, or a mix of them. An incomplete first measure is called a pickup.
What it sounds like
You hear a measure as a regular “heavy” beat: one, two, three, four, one… The bar line itself is silent, but you feel where it stands.
Where it is taught: the studio’s interactive textbook, chapter 3, “Measures and time signatures”.