Full Idea
The realisation that brought 'i' into the toolkit of physicists and engineers was that you could extend the 'number line' into a new dimension, with an imaginary number axis at right angles to it. ...We now have a 'number plane'.
Gist of Idea
By adding an axis of imaginary numbers, we get the useful 'number plane' instead of number line
Source
Brian Clegg (Infinity: Quest to Think the Unthinkable [2003], Ch.12)
Book Reference
Clegg,Brian: 'Infinity' [Robinson 2003], p.163