An ordinal number tells you where something is in a sequence of things. For example the sixth house on a street, the third row of seats.
The word is derived from the idea of 'order'. When things are arranged in some known order, then the ordinal is the position in that sequence.
For this to work, we must know the way the items are ordered. For example if we arrange children by age, then child 1 (the first one) is the youngest, followed by child 2 and child 3 etc. These "child numbers" are the ordinal numbers of the set of children when put in age order.
If we put the children in height order, then child one (the first one) would be the shortest etc.
As we drive along many major roads we see that the junctions are often numbered.
The designers start at one end and the first intersection (junction) is numbered one, then two etc.
In the above example, starting on the left (west) end of the road there are five junctions.
So the first junction is junction 1, the second is junction 2 etc. These "junction numbers" are the ordinal numbers of the junctions.
Arithmetic with ordinal numbers
There is a limited way that ordinal numbers can be worked with arithmetic. For example if I tell you to go to junction 2, then go three more junctions, you can take the 2, add 3 and get junction 5. Aside from situations like this, there is little meaningful arithmetic that can be done with ordinal numbers.
While you are here..
... I have a small favor to ask. Over the years we have used advertising to support the site so it can remain free for everyone.
However, advertising revenue is falling and I have always hated the ads. So, would you go to Patreon and become a patron of the site?
When we reach the goal I will remove all advertising from the site.
It only takes a minute and any amount would be greatly appreciated.
Thank you for considering it! – John Page
Become a patron of the site at patreon.com/mathopenref
Other number topics
Numbers that have factors
(C) 2011 Copyright Math Open Reference. All rights reserved