Markdown consists of blocks of content,
optionally prepended a set of Metadata blocks.
Visually, this can be described using a syntax diagram
where the possible order of elements are laid out
like trains on rails,
as seen in @fig-def-Markdown and @fig-def-Block.

![Markdown](def_Markdown.svg){#fig-def-Markdown}

![Block](def_Block.svg){#fig-def-Block}

Reading order matter.
These syntax diagrams should be read left-to-right and top-to-bottom,
also at places with choice --
e.g. the block type `Header` should be tried before `Paragraph`,
since (as elaborated below) a paragraph begins with any words,
including the initial words defitive for other block types.
In other words,
these syntax diagrams do not reflect the more common EBNF grammars,
but instead a parsing expression grammar [@Ford2004],
because context-free grammars are unlikely to cover Markdown
[@MacFarlane2014].
The grammar is included as [Appendix @sec-def-peg].

The most common content block is a paragraph,
which consists of lines of space-delimited words
followed by two or more line breaks.

![Paragraph](def_Paragraph.svg){#fig-def-Paragraph}

Words are sets of printable characters
(including punctuation and other printable characters).
they can be styled
(@fig-def-StyledWords),
have a hyperlink attached
(@fig-def-LinkedWords)
and have annotations attached
(@fig-def-AnnotatedWords).

![StyledWords](def_StyledWords.svg){#fig-def-StyledWords}

![LinkedWords](def_LinkedWords.svg){#fig-def-LinkedWords}

![AnnotatedWords](def_AnnotatedWords.svg){#fig-def-AnnotatedWords}

![PlainWords](def_PlainWords.svg){#fig-def-PlainWords}

Other content blocks include a header
consisting of words
(@fig-def-Header),
and a list consisting of list items,
each containing a block
(@fig-def-List).

![Header](def_Header.svg){#fig-def-Header}

![List](def_List.svg){#fig-def-List}

Yet other content blocks and inline types exist.
Those are omitted in this description,
which is limited to the comonents affected
by extending the Markdown language with additional types of annotation.

Syntax diagrams for additional Markdown components are included
as [Appendix @sec-def-dia].