While not completely related to our Phi explanation and exercise, (we'll be back to that soon, I promise,) explaining ems is absolutely relevant to typography and especially, to responsive typography.

In the old times of manual typesetting, the line-height of the physical block that the letter was built upon was the em. This gave birth to the element knowledge and arbitrary measure of it. Yes, it's arbitrary because it varies from font to font, usually being defined as the width of the capital M (hence the em name,) since the M usually fulfilled the casted blocks width. One em was equal to the current point size – so 1em generically equals 1pt.

In CSS, the em is equal to the height of the font. Since a standard needed to be developed for all browser rules and rendering options, 1em lost its arbitrary measure, being equal to a readable standard of 16pt for the screen. Height still varies depending on the font being used, but writing 1em without any user specific declaration will...