diff --git a/library/core/src/char/methods.rs b/library/core/src/char/methods.rs
index ce33fd1d8f9d..6f6557510243 100644
--- a/library/core/src/char/methods.rs
+++ b/library/core/src/char/methods.rs
@@ -1932,6 +1932,9 @@ pub const fn is_ascii_graphic(&self) -> bool {
     /// U+0020 SPACE, U+0009 HORIZONTAL TAB, U+000A LINE FEED,
     /// U+000C FORM FEED, or U+000D CARRIAGE RETURN.
     ///
+    /// **Warning:** Because the list above excludes U+000B VERTICAL TAB,
+    /// `c.is_ascii_whitespace()` is **not** equivalent to `c.is_ascii() && c.is_whitespace()`.
+    ///
     /// Rust uses the WhatWG Infra Standard's [definition of ASCII
     /// whitespace][infra-aw]. There are several other definitions in
     /// wide use. For instance, [the POSIX locale][pct] includes
diff --git a/library/core/src/num/mod.rs b/library/core/src/num/mod.rs
index 5ba3695bc45c..e305cff31189 100644
--- a/library/core/src/num/mod.rs
+++ b/library/core/src/num/mod.rs
@@ -1065,6 +1065,9 @@ pub const fn is_ascii_graphic(&self) -> bool {
     /// U+0020 SPACE, U+0009 HORIZONTAL TAB, U+000A LINE FEED,
     /// U+000C FORM FEED, or U+000D CARRIAGE RETURN.
     ///
+    /// **Warning:** Because the list above excludes U+000B VERTICAL TAB,
+    /// `b.is_ascii_whitespace()` is **not** equivalent to `char::from(b).is_whitespace()`.
+    ///
     /// Rust uses the WhatWG Infra Standard's [definition of ASCII
     /// whitespace][infra-aw]. There are several other definitions in
     /// wide use. For instance, [the POSIX locale][pct] includes
diff --git a/library/core/src/slice/ascii.rs b/library/core/src/slice/ascii.rs
index edf058c96a52..9db07d8abbbe 100644
--- a/library/core/src/slice/ascii.rs
+++ b/library/core/src/slice/ascii.rs
@@ -222,7 +222,11 @@ pub fn escape_ascii(&self) -> EscapeAscii<'_> {
     /// Returns a byte slice with leading ASCII whitespace bytes removed.
     ///
     /// 'Whitespace' refers to the definition used by
-    /// [`u8::is_ascii_whitespace`].
+    /// [`u8::is_ascii_whitespace`]. Importantly, this definition excludes
+    /// the `\0x0B` byte even though it has the Unicode [`White_Space`] property
+    /// and is removed by [`str::trim_start`].
+    ///
+    /// [`White_Space`]: https://www.unicode.org/reports/tr44/#White_Space
     ///
     /// # Examples
     ///
@@ -251,7 +255,11 @@ pub const fn trim_ascii_start(&self) -> &[u8] {
     /// Returns a byte slice with trailing ASCII whitespace bytes removed.
     ///
     /// 'Whitespace' refers to the definition used by
-    /// [`u8::is_ascii_whitespace`].
+    /// [`u8::is_ascii_whitespace`]. Importantly, this definition excludes
+    /// the `\0x0B` byte even though it has the Unicode [`White_Space`] property
+    /// and is removed by [`str::trim_end`].
+    ///
+    /// [`White_Space`]: https://www.unicode.org/reports/tr44/#White_Space
     ///
     /// # Examples
     ///
@@ -281,7 +289,11 @@ pub const fn trim_ascii_end(&self) -> &[u8] {
     /// removed.
     ///
     /// 'Whitespace' refers to the definition used by
-    /// [`u8::is_ascii_whitespace`].
+    /// [`u8::is_ascii_whitespace`]. Importantly, this definition excludes
+    /// the `\0x0B` byte even though it has the Unicode [`White_Space`] property
+    /// and is removed by [`str::trim`].
+    ///
+    /// [`White_Space`]: https://www.unicode.org/reports/tr44/#White_Space
     ///
     /// # Examples
     ///
diff --git a/library/core/src/str/mod.rs b/library/core/src/str/mod.rs
index 1a4493de30a1..73fb4c6b2c87 100644
--- a/library/core/src/str/mod.rs
+++ b/library/core/src/str/mod.rs
@@ -1202,6 +1202,9 @@ pub fn split_whitespace(&self) -> SplitWhitespace<'_> {
     ///
     /// This uses the same definition as [`char::is_ascii_whitespace`].
     /// To split by Unicode `Whitespace` instead, use [`split_whitespace`].
+    /// Note that because of this difference in definition, even if `s.is_ascii()`
+    /// is `true`, `s.split_ascii_whitespace()` behavior will differ from `s.split_whitespace()`
+    /// if `s` contains U+000B VERTICAL TAB.
     ///
     /// [`split_whitespace`]: str::split_whitespace
     ///
@@ -2896,9 +2899,12 @@ pub const fn make_ascii_lowercase(&mut self) {
     /// Returns a string slice with leading ASCII whitespace removed.
     ///
     /// 'Whitespace' refers to the definition used by
-    /// [`u8::is_ascii_whitespace`].
+    /// [`u8::is_ascii_whitespace`]. Importantly, this definition excludes
+    /// the U+000B code point even though it has the Unicode [`White_Space`] property
+    /// and is removed by [`str::trim_start`].
     ///
     /// [`u8::is_ascii_whitespace`]: u8::is_ascii_whitespace
+    /// [`White_Space`]: https://www.unicode.org/reports/tr44/#White_Space
     ///
     /// # Examples
     ///
@@ -2921,9 +2927,12 @@ pub const fn trim_ascii_start(&self) -> &str {
     /// Returns a string slice with trailing ASCII whitespace removed.
     ///
     /// 'Whitespace' refers to the definition used by
-    /// [`u8::is_ascii_whitespace`].
+    /// [`u8::is_ascii_whitespace`]. Importantly, this definition excludes
+    /// the U+000B code point even though it has the Unicode [`White_Space`] property
+    /// and is removed by [`str::trim_end`].
     ///
     /// [`u8::is_ascii_whitespace`]: u8::is_ascii_whitespace
+    /// [`White_Space`]: https://www.unicode.org/reports/tr44/#White_Space
     ///
     /// # Examples
     ///
@@ -2947,9 +2956,12 @@ pub const fn trim_ascii_end(&self) -> &str {
     /// removed.
     ///
     /// 'Whitespace' refers to the definition used by
-    /// [`u8::is_ascii_whitespace`].
+    /// [`u8::is_ascii_whitespace`]. Importantly, this definition excludes
+    /// the U+000B code point even though it has the Unicode [`White_Space`] property
+    /// and is removed by [`str::trim`].
     ///
     /// [`u8::is_ascii_whitespace`]: u8::is_ascii_whitespace
+    /// [`White_Space`]: https://www.unicode.org/reports/tr44/#White_Space
     ///
     /// # Examples
     ///