A shopkeeper sells $x$ kg of rice and $y$ kg of wheat, 40 kg in all, so $x+y=40$. Where does the graph of this equation meet the $y$-axis, and what does that point mean?

**$x$-axis**: every point on the $x$-axis has $y=0$. So the $x$-axis is the line **$y=0\ast \ast .\ast \ast y$-axis**: every point on the $y$-axis has $x=0$. So the $y$-axis is the line **$x=0$**.

These are special cases of 'lines parallel to the axes' — except they ARE the axes themselves.