Not everything can be measured in whole numbers and it is very advantageous to be able to use what are called fractions. 

We all use fractions; we talk about half of something, a quarter, even our measuring system (the metric system) uses tenths, hundredths and thousandths.

We find fractions everywhere and we normally define a fraction as part of a whole.

A fraction is written in two parts like this example:

The top number is called the numerator and the bottom number is called the denominator. The horizontal line is called the vinculum.

We read this fraction as five-eighths, five over eight or five divided by eight.

The denominator tells us how many parts make up the whole and the numerator tells us how many of those parts we have.  So in our example above, something has been divided into eight equal parts and we have five of those parts.

So why are they called common fractions?  I suppose because they are just the normal type of fraction we meet everyday.  When we say a fraction is a common fraction, it means that both the numerator and denominator are whole numbers. (More correctly, we should say integers, which means that the numbers could be negative.)

Sometime, you might even hear them referred to as proper fractions or vulgar fractions ... doesn't sound very nice, does it!  However the names don't stop there!

There is the improper fraction like

which means that the numerator is bigger than the denominator; and then there's the mixed fraction like

which is a combination of a proper fraction and a whole number.

Okay, enough of the names, let's see how our trusty slide rule can help us out with fractions!