There are 5 6 NINE (or 63*, depending how you count) different ways to write multi-line strings in YAML.

TL;DR

• Use > most of the time: interior line breaks are stripped out, although you get one at the end:

    key: >
      Your long
      string here.
  

• Use | if you want those linebreaks to be preserved as n (for instance, embedded markdown with paragraphs).

    key: |
      ### Heading
  
      * Bullet
      * Points
  

• Use >- or |- instead if you don’t want a linebreak appended at the end.

• Use "..." if you need to split lines in the middle of words or want to literally type linebreaks as n:

    key: "Antidisestab
     lishmentarianism.nnGet on it."
  

• YAML is crazy.

Block scalar styles (>, |)

These allow characters such as and " without escaping, and add a new line (n) to the end of your string.

> Folded style removes single newlines within the string (but adds one at the end, and converts double newlines to singles):

Key: >
  this is my very very very
  long string

→ this is my very very very long stringn

Extra leading space is retained and causes extra newlines. See note below.

Advice: Use this. Usually this is what you want.

| Literal style
turns every newline within the string into a literal newline, and adds one at the end:

Key: |
  this is my very very very 
  long string

→ this is my very very verynlong stringn

Here’s the official definition from the YAML Spec 1.2

Scalar content can be written in block notation, using a literal style (indicated by “|”) where all line breaks are significant. Alternatively, they can be written with the folded style (denoted by “>”) where each line break is folded to a space unless it ends an empty or a more-indented line.

Advice: Use this for inserting formatted text (especially Markdown) as a value.

Block styles with block chomping indicator (>-, |-, >+, |+)

You can control the handling of the final new line in the string, and any trailing blank lines (nn) by adding a block chomping indicator character:

• >, |: “clip”: keep the line feed, remove the trailing blank lines.
• >-, |-: “strip”: remove the line feed, remove the trailing blank lines.
• >+, |+: “keep”: keep the line feed, keep trailing blank lines.

“Flow” scalar styles ( , ", ')

These have limited escaping, and construct a single-line string with no new line characters. They can begin on the same line as the key, or with additional newlines first, which are stripped. Doubled newline characters become one newline.

plain style (no escaping, no # or : combinations, first character can’t be ", ' or many other punctuation characters ):

Key: this is my very very very 
  long string

Advice: Avoid. May look convenient, but you’re liable to shoot yourself in the foot by accidentally using forbidden punctuation and triggering a syntax error.

double-quoted style ( and " must be escaped by , newlines can be inserted with a literal n sequence, lines can be concatenated without spaces with trailing ):

Key: "this is my very very "very" loooo
  ng string.nnLove, YAML."

→ "this is my very very "very" loooong string.nnLove, YAML."

Advice: Use in very specific situations. This is the only way you can break a very long token (like a URL) across lines without adding spaces. And maybe adding newlines mid-line is conceivably useful.

single-quoted style (literal ' must be doubled, no special characters, possibly useful for expressing strings starting with double quotes):

Key: 'this is my very very "very"
  long string, isn''t it.'

→ "this is my very very "very" long string, isn't it."

Advice: Avoid. Very few benefits, mostly inconvenience.

Block styles with indentation indicators

Just in case the above isn’t enough for you, you can add a “block indentation indicator” (after your block chomping indicator, if you have one):

- >8
        My long string
        starts over here
- |+1
 This one
 starts here

Note: Leading spaces in Folded style (>)

If you insert extra spaces at the start of not-the-first lines in Folded style, they will be kept, with a bonus newline. (This doesn’t happen with flow styles.) Section 6.5:

In addition, folding does not apply to line breaks surrounding text lines that contain leading white space. Note that such a more-indented line may consist only of such leading white space.

- >
    my long
      string
                    
    many spaces above
- my long
      string
                    
    many spaces above
    

→ ["my longn stringn nmany spaces aboven","my long stringnmany spaces above"]

Summary

In this table: _ means space character, n means “newline character” except were noted. “Leading space” refers to an additional space character on the second line, when the first is only spaces (which establishes the indent).

Examples

Note the trailing spaces on the line before “spaces.”

- >
  very "long"
  'string' with

  paragraph gap, n and        
  spaces.
- | 
  very "long"
  'string' with

  paragraph gap, n and        
  spaces.
- very "long"
  'string' with

  paragraph gap, n and        
  spaces.
- "very "long"
  'string' with

  paragraph gap, n and        
  s
  p
  a
  c
  e
  s."
- 'very "long"
  ''string'' with

  paragraph gap, n and        
  spaces.'
- >- 
  very "long"
  'string' with

  paragraph gap, n and        
  spaces.

[
  "very "long" 'string' withnparagraph gap, \n and         spaces.n", 
  "very "long"n'string' withnnparagraph gap, \n and        nspaces.n", 
  "very "long" 'string' withnparagraph gap, \n and spaces.", 
  "very "long" 'string' withnparagraph gap, n and spaces.", 
  "very "long" 'string' withnparagraph gap, \n and spaces.", 
  "very "long" 'string' withnparagraph gap, \n and         spaces."
]

*2 block styles, each with 2 possible block chomping indicators (or none), and with 9 possible indentation indicators (or none), 1 plain style and 2 quoted styles: 2 x (2 + 1) x (9 + 1) + 1 + 2 = 63

Some of this information has also been summarised here.

Using yaml folded style. The indention in each line will be ignored. A line break will be inserted at the end.

Key: >
  This is a very long sentence
  that spans several lines in the YAML
  but which will be rendered as a string
  with only a single carriage return appended to the end.

http://symfony.com/doc/current/components/yaml/yaml_format.html

You can use the “block chomping indicator” to eliminate the trailing line break, as follows:

Key: >-
  This is a very long sentence
  that spans several lines in the YAML
  but which will be rendered as a string
  with NO carriage returns.

In either case, each line break is replaced by a space.

There are other control tools available as well (for controlling indentation for example).

See https://yaml-multiline.info/

To preserve newlines use |, for example:

Key: |
  This is a very long sentence
  that spans several lines in the YAML
  but which will be rendered as a string
  with newlines preserved.

is translated to “This is a very long sentence‌**n** that spans several lines in the YAML‌**n** but which will be rendered as a string‌**n** with newlines preserved.n“

1. Block Notation(plain, flow-style, scalar): Newlines become spaces and extra newlines after the block are removed

---
# Note: It has 1 new line after the string
content:
    Arbitrary free text
    over multiple lines stopping
    after indentation changes...

...

Equivalent JSON

{
 "content": "Arbitrary free text over multiple lines stopping after indentation changes..."
}

2. Literal Block Scalar: A Literal Block Scalar | will include the newlines and any trailing spaces. but removes extra

newlines after the block.

---
# After string we have 2 spaces and 2 new lines
content1: |
 Arbitrary free text
 over "multiple lines" stopping
 after indentation changes...  


...

Equivalent JSON

{
 "content1": "Arbitrary free textnover "multiple lines" stoppingnafter indentation changes...  n"
}

3. + indicator with Literal Block Scalar: keep extra newlines after block

---
# After string we have 2 new lines
plain: |+
 This unquoted scalar
 spans many lines.


...

Equivalent JSON

{
 "plain": "This unquoted scalarnspans many lines.nnn"
}

4. – indicator with Literal Block Scalar: – means that the newline at the end of the string is removed.

---
# After string we have 2 new lines
plain: |-
 This unquoted scalar
 spans many lines.


...

Equivalent JSON

{
 "plain": "This unquoted scalarnspans many lines."
}

5. Folded Block Scalar(>):

will fold newlines to spaces and but removes extra newlines after the block.

---
folded_newlines: >
 this is really a
 single line of text
 despite appearances


...

Equivalent JSON

{
 "fold_newlines": "this is really a single line of text despite appearancesn"
}

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To concatenate long lines without whitespace, use double quotes and escape the newlines with backslashes:

key: "Loremipsumdolorsitamet,consecteturadipiscingelit,seddoeiusmodtemp
  orincididuntutlaboreetdoloremagnaaliqua."

You might not believe it, but YAML can do multi-line keys too:

?
 >
 multi
 line
 key
:
  value

In case you’re using YAML and Twig for translations in Symfony, and want to use multi-line translations in Javascript, a carriage return is added right after the translation. So even the following code:

var javascriptVariable = "{{- 'key'|trans -}}";

Which has the following yml translation:

key: >
    This is a
    multi line 
    translation.

Will still result into the following code in html:

var javascriptVariable = "This is a multi line translation.
";

So, the minus sign in Twig does not solve this. The solution is to add this minus sign after the greater than sign in yml:

key: >-
    This is a
    multi line 
    translation.

Will have the proper result, multi line translation on one line in Twig:

var javascriptVariable = "This is a multi line translation.";

For situations were the string might contain spaces or not, I prefer double quotes and line continuation with backslashes:

key: "String 
  with long c
  ontent"

But note about the pitfall for the case that a continuation line begins with a space, it needs to be escaped (because it will be stripped away elsewhere):

key: "String
   with lon
  g content"

If the string contains line breaks, this needs to be written in C style n.

See also this question.

None of the above solutions worked for me, in a YAML file within a Jekyll project. After trying many options, I realized that an HTML injection with <br> might do as well, since in the end everything is rendered to HTML:

name: |
In a village of La Mancha <br> whose name I don’t <br> want to remember.

At least it works for me. No idea on the problems associated to this approach.