type-foundationslisted

# Type foundations The vocabulary and measurement system everything else depends on. Most typographic mistakes at the decision level trace back to a wrong model here. ## 1. When to invoke - A term needs defining before a decision can be made. - Someone conflates font/typeface, leading/line-height, or point/pixel. - Choosing CSS units for type. - Explaining a typographic decision to a developer or client. **Do not** invoke when the decision is already scoped — go to the specific skill. ## 2. Required context - The **medium** — print units and screen units are related but not equivalent. - The audience's **existing vocabulary**, so you correct only what matters. ## 3. Invariant principles **Typeface vs. font.** A *typeface* is the design (Helvetica). A *font* is a specific instance of it that you use (Helvetica Bold 12pt; a `.woff2` file). Historically a font was one size of metal type. The distinction matters in licensing and in file inventories. **Glyph vs. character.** A *character* is the abstract unit (the letter "a"); a *glyph* is a drawn form. One character may have many glyphs (small cap, oldstyle, alternate); one glyph may represent several characters (the `fi` ligature). **The em square is the design space.** An em is not the width of "M" — it's the full body of the type, historically the metal block. Glyphs are drawn within it and may fill it generously or sparsely, which is why **two typefaces at the same point size render at visibly different apparent siz