The Baseline (Vertical Align, Line Height, etc.)

In typography, the baseline is a line the letter sit on. If you've never heard this term, read the Wikipedia entry) quickly.

line-height describes the amount of space between the baselines.

As you probably guessed, the boxes are inline-blocks. Inline elements sit on the baseline, but you can adjust how they set using the vertical-align property. If you were to look this up on MDN, you'll have the default is baseline, but you can make it hang in the middle or the top accordingly.