Today's theory lesson will revolve around the measure (US) or bar (UK) both terms being acceptable, and follows on from the previous lesson on staffs, clefs & ledger lines.
What is a bar or measure?
Firstly, let’s get the naming issue out of the way; a bar is simply the U.K name and a measure is the U.S name for the same thing, although the term "bar" is understood by both so I will use this term. So, a bar is a small section of music signified by a vertical line or lines written on the staff that divides it into equal note counts according to the time signature. For instance, a time signature of 4/4 indicates each bar should be no more, or no less than four quarter notes, whatever the range of note values inside that bar. In other words, it may be two quarter notes and four eighth notes that make up the bar, but it still adds-up to four quarter notes.
The example piece of music below shows how the addition of note values works, along with some bar lines including a repeat start and finish lines.
Types of bars
Standard bar line
There are a number of different types of bar lines, starting with our standard single vertical bar line which separates each bar into the time signatures value. The standard bar does not infer anything other than continuation of the musical piece.
Double bar line
Next is our double bar line which is two vertical lines side by side. A double bar line is used as a separator between two musical sections, for instance, a verse and a chorus. A double bar line may also be used to for a key change that occurs anywhere other than a new section of music (new staff).
End bar line
Our next bar line is called an end bar line and is strictly used to end a musical score, ie: the end of a musical piece and should be placed at the very last bar.
Repeat start & end bar
Next is what we call a repeat start bar and a repeat end bar which must be paired together, although the amount of music in between does not matter. From the repeat start bar you play the music until you reach the repeat end bar, at which point you go back to the repeat start bar and play right through to the repeat end bar again, then continue on with the rest of the musical piece. It is frequent to find a x4 sign above the repeat end bar which signifies to play the repeat four times, or whatever the value is.
Free time bar
And lastly is the free time bar which is used in place of a standard bar whenever the tempo of a musical piece is notated as "Free Time".
A bar or measure gives a musical piece its structure, and a composer the tool he/she needs to form that structure in order to relay his musical composition. Try to think of a bar as something that divides a piece of music into logical parts or sections.
Cheers & enjoy!