x-axis and y-axis – Naming the Two Number Lines
The two number lines that make up the coordinate plane each have their own name. The x-axis is the horizontal number line, running left to right. The y-axis is the vertical number line, running up and down. Descartes originally only used a single horizontal axis in his 1637 work – it was later mathematicians, refining his ideas over the following century, who standardised the second, vertical axis and the x/y labelling we use today.
A useful memory trick: “x is across” – both words start with a similar side-to-side feel, and the x-axis is the one you read left-to-right, just like text. Once x and y are locked in as automatic reflexes, every coordinate you read or plot for the rest of this section becomes much faster.
Telling the Axes Apart
The x-axis is horizontal (left-right). The y-axis is vertical (up-down). A point sits on the x-axis when its y-value is 0, and on the y-axis when its x-value is 0.
Its x-value is 0, so it lies on the y-axis, seven units above the origin.
Its y-value is 0, so it lies on the x-axis, four units to the left of the origin.
Real-Life Application
- Graphs: almost every chart or graph you'll ever read is built on an x-axis and a y-axis.
- Spreadsheets: chart axes in spreadsheet software follow the same convention.
- Engineering drawings: horizontal and vertical reference lines guide precise measurements.
Key Takeaways
- The x-axis is horizontal; the y-axis is vertical.
- A point is on the x-axis when y = 0, and on the y-axis when x = 0.
- The x/y labelling was standardised after Descartes's original 1637 work.
Practice: x-axis and y-axis
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