Headings — # Through ######

Headings define the document outline

Markdown supports six heading levels using # (ATX-style). Headings are not decoration — they are the document outline. Screen readers, table-of-contents generators, and search engines all depend on them.

Hard rules

• A space is required after the # characters. #heading with no space is not a heading in CommonMark.
• You can optionally close headings: ## Section ##. The closing hashes are stripped.
• Up to 6 levels — anything past ###### is just literal hashes.

Best practice

Use # H1 exactly once per document — that is your title. Structure the rest with ## for major sections and ### for subsections. Do not skip levels (H2 → H4 confuses screen readers and breaks auto-generated TOCs).

Setext-style — the alternate H1/H2 syntax

Underline text with = for H1 and - for H2. Older syntax, still valid, but ATX is more common because it works for all six levels and is easier to skim in source.