diff --git a/encoding.c b/encoding.c
index 3be39d8e09..7bca8d1b2b 100644
--- a/encoding.c
+++ b/encoding.c
@@ -1988,6 +1988,7 @@ Init_unicode_version(void)
     VALUE str = rb_usascii_str_new_static(onigenc_unicode_version_string,
                                           strlen(onigenc_unicode_version_string));
     OBJ_FREEZE(str);
+    /* The supported Unicode version. */
     rb_define_const(rb_cEncoding, "UNICODE_VERSION", str);
 }
 
diff --git a/error.c b/error.c
index d3aeec54d8..e14ecd2393 100644
--- a/error.c
+++ b/error.c
@@ -3499,6 +3499,18 @@ syserr_eqq(VALUE self, VALUE exc)
  *  incompatible with the target encoding.
  */
 
+/*
+ *  Document-class: NoMatchingPatternError
+ *
+ *  Raised when matching pattern not found.
+ */
+
+/*
+ *  Document-class: NoMatchingPatternKeyError
+ *
+ *  Raised when matching key not found.
+ */
+
 /*
  * Document-class: fatal
  *
diff --git a/namespace.c b/namespace.c
index e667590890..55b7580c72 100644
--- a/namespace.c
+++ b/namespace.c
@@ -411,6 +411,12 @@ rb_get_namespace_object(rb_namespace_t *ns)
 
 static void setup_pushing_loading_namespace(rb_namespace_t *ns);
 
+/*
+ *  call-seq:
+ *    Namespace.new -> new_namespace
+ *
+ *  Returns a new Namespace object.
+ */
 static VALUE
 namespace_initialize(VALUE namespace)
 {
@@ -450,12 +456,26 @@ namespace_initialize(VALUE namespace)
     return namespace;
 }
 
+/*
+ *  call-seq:
+ *    Namespace.enabled? -> true or false
+ *
+ *  Returns +true+ if namespace is enabled.
+ */
 static VALUE
 rb_namespace_s_getenabled(VALUE namespace)
 {
     return RBOOL(rb_namespace_available());
 }
 
+/*
+ *  call-seq:
+ *    Namespace.current -> namespace, nil or false
+ *
+ *  Returns the current namespace.
+ *  Returns +nil+ if it is the built-in namespace.
+ *  Returns +false+ if namespace is not enabled.
+ */
 static VALUE
 rb_namespace_current(VALUE klass)
 {
@@ -469,6 +489,12 @@ rb_namespace_current(VALUE klass)
     return Qfalse;
 }
 
+/*
+ *  call-seq:
+ *    Namespace.is_builtin?(klass) -> true or false
+ *
+ *  Returns +true+ if +klass+ is only in a user namespace.
+ */
 static VALUE
 rb_namespace_s_is_builtin_p(VALUE namespace, VALUE klass)
 {
@@ -477,6 +503,12 @@ rb_namespace_s_is_builtin_p(VALUE namespace, VALUE klass)
     return Qfalse;
 }
 
+/*
+ *  call-seq:
+ *    load_path -> array
+ *
+ *  Returns namespace local load path.
+ */
 static VALUE
 rb_namespace_load_path(VALUE namespace)
 {
@@ -1048,6 +1080,13 @@ Init_enable_namespace(void)
     }
 }
 
+/*
+ *  Document-class: Namespace
+ *
+ *  Namespace is designed to provide separated spaces in a Ruby
+ *  process, to isolate applications and libraries.
+ *  See {Namespace}[rdoc-ref:namespace.md].
+ */
 void
 Init_Namespace(void)
 {
@@ -1057,14 +1096,17 @@ Init_Namespace(void)
     rb_cNamespace = rb_define_class("Namespace", rb_cModule);
     rb_define_method(rb_cNamespace, "initialize", namespace_initialize, 0);
 
+    /* :nodoc: */
     rb_cNamespaceEntry = rb_define_class_under(rb_cNamespace, "Entry", rb_cObject);
     rb_define_alloc_func(rb_cNamespaceEntry, rb_namespace_entry_alloc);
 
+    /* :nodoc: */
     rb_mNamespaceRefiner = rb_define_module_under(rb_cNamespace, "Refiner");
     if (rb_namespace_available()) {
         setup_builtin_refinement(rb_mNamespaceRefiner);
     }
 
+    /* :nodoc: */
     rb_mNamespaceLoader = rb_define_module_under(rb_cNamespace, "Loader");
     namespace_define_loader_method("require");
     namespace_define_loader_method("require_relative");
diff --git a/pathname_builtin.rb b/pathname_builtin.rb
index fac0b46d9a..95bd8d0e3a 100644
--- a/pathname_builtin.rb
+++ b/pathname_builtin.rb
@@ -12,6 +12,7 @@
 
 class Pathname
 
+  # The version string.
   VERSION = "0.4.0"
 
   # :stopdoc:
diff --git a/prelude.rb b/prelude.rb
index f49cada637..36da381804 100644
--- a/prelude.rb
+++ b/prelude.rb
@@ -15,15 +15,17 @@ class Binding
 end
 
 module Kernel
+  # :stopdoc:
   def pp(*objs)
     require 'pp'
     pp(*objs)
   end
 
   # suppress redefinition warning
-  alias pp pp # :nodoc:
+  alias pp pp
 
   private :pp
+  # :startdoc:
 end
 
 module Enumerable
diff --git a/proc.c b/proc.c
index 8543110476..68f63040b7 100644
--- a/proc.c
+++ b/proc.c
@@ -2026,6 +2026,12 @@ method_owner(VALUE obj)
     return data->owner;
 }
 
+/*
+ *  call-see:
+ *    meth.namespace   -> namespace or nil
+ *
+ *  Returns the namespace where +meth+ is defined in.
+ */
 static VALUE
 method_namespace(VALUE obj)
 {
diff --git a/ractor.c b/ractor.c
index 721234a98b..a46eb00685 100644
--- a/ractor.c
+++ b/ractor.c
@@ -874,6 +874,12 @@ ractor_moved_missing(int argc, VALUE *argv, VALUE self)
     rb_raise(rb_eRactorMovedError, "can not send any methods to a moved object");
 }
 
+/*
+ *  Document-class: Ractor::Error
+ *
+ *  The parent class of Ractor-related error classes.
+ */
+
 /*
  *  Document-class: Ractor::ClosedError
  *
@@ -911,6 +917,13 @@ ractor_moved_missing(int argc, VALUE *argv, VALUE self)
  *     Continue successfully
  */
 
+/*
+ *  Document-class: Ractor::IsolationError
+ *
+ *  Raised on attempt to make a Ractor-unshareable object
+ *  Ractor-shareable.
+ */
+
 /*
  *  Document-class: Ractor::RemoteError
  *
@@ -960,6 +973,12 @@ ractor_moved_missing(int argc, VALUE *argv, VALUE self)
  *     # Ractor::MovedError (can not send any methods to a moved object)
  */
 
+/*
+ *  Document-class: Ractor::UnsafeError
+ *
+ *  Raised when Ractor-unsafe C-methods is invoked by a non-main Ractor.
+ */
+
 // Main docs are in ractor.rb, but without this clause there are weird artifacts
 // in their rendering.
 /*
diff --git a/ractor.rb b/ractor.rb
index ee6135b81e..5827f6672b 100644
--- a/ractor.rb
+++ b/ractor.rb
@@ -612,6 +612,7 @@ class Ractor
     __builtin_ractor_unmonitor(port)
   end
 
+  # \Port objects transmit messages between Ractors.
   class Port
     #
     # call-seq:
diff --git a/ractor_sync.c b/ractor_sync.c
index 8437516bfb..a3ed38295b 100644
--- a/ractor_sync.c
+++ b/ractor_sync.c
@@ -91,12 +91,19 @@ ractor_port_init(VALUE rpv, rb_ractor_t *r)
     return rpv;
 }
 
+/*
+ *  call-seq:
+ *    Ractor::Port.new  -> new_port
+ *
+ *  Returns a new Ractor::Port object.
+ */
 static VALUE
 ractor_port_initialzie(VALUE self)
 {
     return ractor_port_init(self, GET_RACTOR());
 }
 
+/* :nodoc: */
 static VALUE
 ractor_port_initialzie_copy(VALUE self, VALUE orig)
 {
diff --git a/re.c b/re.c
index b47538d594..9348622eea 100644
--- a/re.c
+++ b/re.c
@@ -4855,6 +4855,7 @@ Init_Regexp(void)
     rb_define_method(rb_cRegexp, "named_captures", rb_reg_named_captures, 0);
     rb_define_method(rb_cRegexp, "timeout", rb_reg_timeout_get, 0);
 
+    /* Raised when regexp matching timed out. */
     rb_eRegexpTimeoutError = rb_define_class_under(rb_cRegexp, "TimeoutError", rb_eRegexpError);
     rb_define_singleton_method(rb_cRegexp, "timeout", rb_reg_s_timeout_get, 0);
     rb_define_singleton_method(rb_cRegexp, "timeout=", rb_reg_s_timeout_set, 1);
diff --git a/scheduler.c b/scheduler.c
index 83b9681cc3..ddb205da88 100644
--- a/scheduler.c
+++ b/scheduler.c
@@ -798,7 +798,7 @@ rb_fiber_scheduler_io_read(VALUE scheduler, VALUE io, VALUE buffer, size_t lengt
 }
 
 /*
- *  Document-method: Fiber::Scheduler#io_read
+ *  Document-method: Fiber::Scheduler#io_pread
  *  call-seq: io_pread(io, buffer, from, length, offset) -> read length or -errno
  *
  *  Invoked by IO#pread or IO::Buffer#pread to read +length+ bytes from +io+
@@ -837,7 +837,7 @@ rb_fiber_scheduler_io_pread(VALUE scheduler, VALUE io, rb_off_t from, VALUE buff
 }
 
 /*
- *  Document-method: Scheduler#io_write
+ *  Document-method: Fiber::Scheduler#io_write
  *  call-seq: io_write(io, buffer, length, offset) -> written length or -errno
  *
  *  Invoked by IO#write or IO::Buffer#write to write +length+ bytes to +io+ from
diff --git a/set.c b/set.c
index 61f1fd8bc4..f83fb0880c 100644
--- a/set.c
+++ b/set.c
@@ -406,6 +406,13 @@ set_s_alloc(VALUE klass)
     return set_alloc_with_size(klass, 0);
 }
 
+/*
+ *  call-seq:
+ *    Set[*objects] -> new_set
+ *
+ *  Returns a new Set object populated with the given objects,
+ *  See Set::new.
+ */
 static VALUE
 set_s_create(int argc, VALUE *argv, VALUE klass)
 {
@@ -522,6 +529,7 @@ set_i_initialize(int argc, VALUE *argv, VALUE set)
     return set;
 }
 
+/* :nodoc: */
 static VALUE
 set_i_initialize_copy(VALUE set, VALUE other)
 {