Leading (line-height)
The vertical space between lines of text. Pronounced 'ledding', after the lead strips printers slid between rows of metal type.
See it
What it is
Leading (said 'ledding') is the vertical distance from one baseline to the next. The name comes from the strips of lead a compositor slid between rows of metal type. On the web it is line-height, and you should set it unitless (1.5, not 24px) so every nested element recomputes its own spacing instead of inheriting a frozen pixel value.
Body copy usually wants 1.4 to 1.6. Headlines want less, 1.0 to 1.2, and big display type can drop under 1.0 because the ascenders and descenders already fill the gap. Leading is coupled to line length: the longer the measure, the more leading you need, because the eye has to travel further back to find the start of the next line and a cramped block makes it land on the wrong one.
The gotcha that ruins layouts: browsers split the extra leading in half and put it above and below every line, so a heading with line-height 1.2 carries invisible padding above its caps and below its baseline. That is why your carefully measured 24px gap looks like 32px. Trim it with the newer text-box-trim and text-box-edge properties, or subtract the half-leading with a negative margin.
Ask AI for it
Set the leading for this page with unitless line-height values. Body copy at 16px to 18px: 1.55. Long-form article text on a measure over 70 characters: 1.65. UI labels and buttons: 1.2. Headings 24px to 40px: 1.25. Display headings 48px and up: 1.05. Never set line-height in px, and never let a paragraph exceed 75 characters per line without raising the leading. Then remove the browser's half-leading above and below headings using text-box-trim: trim-both with text-box-edge: cap alphabetic, so the visible gap matches the spacing value.