diff --git a/doc/stringio/each_line.md b/doc/stringio/each_line.md
new file mode 100644
index 0000000..d75d315
--- /dev/null
+++ b/doc/stringio/each_line.md
@@ -0,0 +1,186 @@
+With a block given calls the block with each remaining line (see "Position" below) in the stream;
+returns `self`.
+
+Leaves stream position at end-of-stream.
+
+**No Arguments**
+
+With no arguments given,
+reads lines using the default record separator
+(global variable `$/`, whose initial value is `"\n"`).
+
+```ruby
+strio = StringIO.new(TEXT)
+strio.each_line {|line| p line }
+strio.eof? # => true
+```
+
+Output:
+
+```
+"First line\n"
+"Second line\n"
+"\n"
+"Fourth line\n"
+"Fifth line\n"
+```
+
+**Argument `sep`**
+
+With only string argument `sep` given,
+reads lines using that string as the record separator:
+
+```
+strio = StringIO.new(TEXT)
+strio.each_line(' ') {|line| p line }
+```
+
+Output:
+
+```
+"First "
+"line\nSecond "
+"line\n\nFourth "
+"line\nFifth "
+"line\n"
+```
+
+**Argument `limit`**
+
+With only integer argument `limit` given,
+reads lines using the default record separator;
+also limits the size (in characters) of each line to the given limit:
+
+```
+strio = StringIO.new(TEXT)
+strio.each_line(10) {|line| p line }
+```
+
+Output:
+
+```
+"First line"
+"\n"
+"Second lin"
+"e\n"
+"\n"
+"Fourth lin"
+"e\n"
+"Fifth line"
+"\n"
+```
+**Arguments `sep` and `limit`**
+
+With arguments `sep` and `limit` both given,
+honors both:
+
+```
+strio = StringIO.new(TEXT)
+strio.each_line(' ', 10) {|line| p line }
+```
+
+Output:
+
+```
+"First "
+"line\nSecon"
+"d "
+"line\n\nFour"
+"th "
+"line\nFifth"
+" "
+"line\n"
+```
+
+**Position**
+
+As stated above, method `each` _remaining_ line in the stream.
+
+In the examples above each `strio` object starts with its position at beginning-of-stream;
+but in other cases the position may be anywhere (see StringIO#pos):
+
+```
+strio = StringIO.new(TEXT)
+strio.pos = 30 # Set stream position to character 30.
+strio.each_line {|line| p line }
+```
+
+Output:
+
+```
+" line\n"
+"Fifth line\n"
+```
+
+In all the examples above, the stream position is at the beginning of a character;
+in other cases, that need not be so:
+
+```ruby
+s = 'こんにちは'  # Five 3-byte characters.
+strio = StringIO.new(s)
+strio.pos = 3   # At beginning of second character.
+strio.each_line {|line| p line }
+strio.pos = 4   # At second byte of second character.
+strio.each_line {|line| p line }
+strio.pos = 5   # At third byte of second character.
+strio.each_line {|line| p line }
+```
+
+Output:
+
+"んにちは"
+"\x82\x93にちは"
+"\x93にちは"
+```
+
+**Special Record Separators**
+
+Like some methods in class `IO`, StringIO.each honors two special record separators;
+see {Special Line Separators}[https://docs.ruby-lang.org/en/master/IO.html#class-IO-label-Special+Line+Separator+Values].
+
+```
+strio = StringIO.new(TEXT)
+strio.each_line('') {|line| p line } # Read as paragraphs (separated by blank lines).
+```
+
+Output:
+
+```
+"First line\nSecond line\n\n"
+"Fourth line\nFifth line\n"
+```
+
+```
+strio = StringIO.new(TEXT)
+strio.each_line(nil) {|line| p line } # "Slurp"; read it all.
+```
+
+Output:
+
+```
+"First line\nSecond line\n\nFourth line\nFifth line\n"
+```
+
+**Keyword Argument `chomp`**
+
+With keyword argument `chomp` given as `true` (the default is `false`),
+removes trailing newline (if any) from each line:
+
+```
+strio = StringIO.new(TEXT)
+strio.each_line(chomp: true) {|line| p line }
+```
+
+Output:
+
+```
+"First line"
+"Second line"
+""
+"Fourth line"
+"Fifth line"
+```
+
+With no block given, returns a new {Enumerator}[https://docs.ruby-lang.org/en/master/Enumerator.html].
+
+Related: StringIO.each_byte, StringIO.each_char, StringIO.each_codepoint.
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 874a3a1..05772aa 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -1466,170 +1466,8 @@ strio_readline(int argc, VALUE *argv, VALUE self)
  *   each_line(limit, chomp: false) {|line| ... }      -> self
  *   each_line(sep, limit, chomp: false) {|line| ... } -> self
  *
- * With a block given calls the block with each remaining line (see "Position" below) in the stream;
- * returns `self`.
+ * :include: stringio/each_line.md
  *
- * Leaves stream position as end-of-stream.
- *
- * **No Arguments**
- *
- * With no arguments given,
- * reads lines using the default record separator global variable `$/`, whose initial value is `"\n"`.
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line {|line| p line }
- * strio.eof? # => true
- * ```
- *
- * Output:
- *
- * ```
- * "First line\n"
- * "Second line\n"
- * "\n"
- * "Fourth line\n"
- * "Fifth line\n"
- * ```
- *
- * **Argument `sep`**
- *
- * With only string argument `sep` given,
- * reads lines using that string as the record separator:
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line(' ') {|line| p line }
- * ```
- *
- * Output:
- *
- * ```
- * "First "
- * "line\nSecond "
- * "line\n\nFourth "
- * "line\nFifth "
- * "line\n"
- * ```
- *
- * **Argument `limit`**
- *
- * With only integer argument `limit` given,
- * reads lines using the default record separator global variable `$/`, whose initial value is `"\n"`;
- * also limits the size (in characters) of each line to the given limit:
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line(10) {|line| p line }
- * ```
- *
- * Output:
- *
- * ```
- * "First line"
- * "\n"
- * "Second lin"
- * "e\n"
- * "\n"
- * "Fourth lin"
- * "e\n"
- * "Fifth line"
- * "\n"
- * ```
- * **Arguments `sep` and `limit`**
- *
- * With arguments `sep` and `limit` both given,
- * honors both:
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line(' ', 10) {|line| p line }
- * ```
- *
- * Output:
- *
- * ```
- * "First "
- * "line\nSecon"
- * "d "
- * "line\n\nFour"
- * "th "
- * "line\nFifth"
- * " "
- * "line\n"
- * ```
- *
- * **Position**
- *
- * As stated above, method `each` _remaining_ line in the stream.
- *
- * In the examples above each `strio` object starts with its position at beginning-of-stream;
- * but in other cases the position may be anywhere (see StringIO#pos):
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.pos = 30 # Set stream position to character 30.
- * strio.each_line {|line| p line }
- * ```
- *
- * Output:
- *
- * ```
- * " line\n"
- * "Fifth line\n"
- * ```
- *
- * **Special Record Separators**
- *
- * Like some methds in class `IO`, StringIO.each honors two special record separators;
- * see {Special Line Separators}[https://docs.ruby-lang.org/en/master/IO.html#class-IO-label-Special+Line+Separator+Values].
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line('') {|line| p line } # Read as paragraphs (separated by blank lines).
- * ```
- *
- * Output:
- *
- * ```
- * "First line\nSecond line\n\n"
- * "Fourth line\nFifth line\n"
- * ```
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line(nil) {|line| p line } # "Slurp"; read it all.
- * ```
- *
- * Output:
- *
- * ```
- * "First line\nSecond line\n\nFourth line\nFifth line\n"
- * ```
- *
- * **Keyword Argument `chomp`**
- *
- * With keyword argument `chomp` given as `true` (the default is `false`),
- * removes trailing newline (if any) from each line:
- *
- * ```
- * strio = StringIO.new(TEXT)
- * strio.each_line(chomp: true) {|line| p line }
- * ```
- *
- * Output:
- *
- * ```
- * "First line"
- * "Second line"
- * ""
- * "Fourth line"
- * "Fifth line"
- * ```
- *
- * With no block given, returns a new {Enumerator}[https://docs.ruby-lang.org/en/master/Enumerator.html].
- *
- * Related: StringIO.each_byte, StringIO.each_char, StringIO.each_codepoint.
  */
 static VALUE
 strio_each(int argc, VALUE *argv, VALUE self)