Line Height (Leading) & Vertical Baseline Rhythm

Line height (historically called leading after the physical lead strips placed between lines of metal type) measures the vertical distance from the baseline of one line to the baseline of the next:

Mathematical Proportions

• Body Copy: Optimal line height is 1.4x to 1.6x (140% to 160%) the font size (e.g., 16px body text requires a 24px line-height). Too little space crowds lines together; too much space disconnects paragraphs.
• Headlines: Because large text carries high visual mass, headlines require tighter relative line height: 1.1x to 1.25x (e.g., 40px headline requires 48px line-height).

Unitless Proportions in CSS

Always specify line-height as a unitless ratio (line-height: 1.5) rather than a fixed pixel value (line-height: 24px), allowing nested children to scale proportionally.