diff --git a/spec/formatting.md b/spec/formatting.md
index 6cc866a688..15d28f9c8c 100644
--- a/spec/formatting.md
+++ b/spec/formatting.md
@@ -92,7 +92,7 @@ and different implementations MAY choose to perform different levels of resoluti
 > or some other locally appropriate value.
 
 Depending on the presence or absence of an _operand_
-and a _function_ or _reserved_ _annotation_,
+and a _function_, _private-use_, or _reserved_ _annotation_,
 one of the following is used to resolve the value of the _expression_:
 
 - If the _expression_ contains no _annotation_,
@@ -100,10 +100,10 @@ one of the following is used to resolve the value of the _expression_:
   depending on the shape of the _operand_.
 - Else, if the _expression_ has a _function_ _annotation_,
   its resolved value is defined by _function resolution_.
-- Else, the _expression_ has a _reserved_ _annotation_.
-  If the _annotation_ uses a `private-start` character that the implementation supports,
-  its value is resolved according to the implementation's specification.
-  Else, an Unsupported Expression error is emitted and a fallback value is used as its value.
+- Else, if the _expression_ has a _private-use_ _annotation_,
+  its resolved value is defined according to the implementations's specification.
+- Else, the _expression_ has a _reserved_ _annotation_,
+  an Unsupported Expression error is emitted and a fallback value is used as its value.
 
 ### Literal Resolution
 
@@ -183,6 +183,8 @@ An _expression_ fails to resolve when:
 
 - A _variable_ _operand_ fails to resolve.
 - A _function_ _annotation_ fails to resolve.
+- A _private-use_ _annotation_ is unsupported by the implementation or if
+  a _private-use_ _annotation_ fails to resolve.
 - The _expression_ has a _reserved_ _annotation_.
 
 The _fallback value_ depends on the contents of the _expression_:
diff --git a/spec/message.abnf b/spec/message.abnf
index 1adfe7a3c9..b38bd2adea 100644
--- a/spec/message.abnf
+++ b/spec/message.abnf
@@ -10,8 +10,8 @@ variant = when 1*(s key) [s] pattern
 key = literal / "*"
 
 expression = "{" [s] ((operand [s annotation]) / annotation) [s] "}"
-operand = literal / variable
-annotation = (function *(s option)) / reserved
+operand    = literal / variable
+annotation = (function *(s option)) / reserved / private-use
 
 literal = quoted / unquoted
 variable = "$" name
@@ -42,11 +42,15 @@ unquoted = unquoted-start *name-char
 unquoted-start = name-start / DIGIT / "."
                / %xB7 / %x300-36F / %x203F-2040
 
-; reserve additional sigils for use by future versions of this
-; specification or for private use by implementations
-reserved       = ( reserved-start / private-start ) reserved-body
-reserved-start = "!" / "@" / "#" / "%" / "*" / "<" / ">" / "/" / "?" / "~"
+
+; reserve sigils for private-use by implementations
+private-use    = private-start reserved-body
 private-start  = "^" / "&"
+
+; reserve additional sigils for use by 
+; future versions of this specification
+reserved       = reserved-start reserved-body
+reserved-start = "!" / "@" / "#" / "%" / "*" / "<" / ">" / "/" / "?" / "~"
 reserved-body  = *( [s] 1*(reserved-char / reserved-escape / literal))
 reserved-char  = %x00-08        ; omit HTAB and LF
                / %x0B-0C        ; omit CR
diff --git a/spec/syntax.md b/spec/syntax.md
index 1cedae38f4..ab8e9bf8f6 100644
--- a/spec/syntax.md
+++ b/spec/syntax.md
@@ -19,7 +19,8 @@
    1. [Variants](#variants)
    1. [Patterns](#patterns)
    1. [Expressions](#expressions)
-      1. [Reserved Sequences](#reserved)
+      1. [Private-Use Sequences](#private-use)
+      2. [Reserved Sequences](#reserved)
 1. [Tokens](#tokens)
    1. [Keywords](#keywords)
    1. [Text](#text)
@@ -410,20 +411,56 @@ option = name [s] "=" [s] (literal / variable)
 > {{+h1 name=above-and-beyond}Above And Beyond{-h1}}
 > ```
 
+#### Private-Use
+
+A **_private-use_** _annotation_ is an _annotation_ whose syntax is reserved
+for use by a specific implementation or by private agreement between multiple implementations. 
+Implementations MAY define their own meaning and semantics for _private-use_ annotations.
+
+A _private-use_ annotation starts with either U+0026 AMPERSAND `&` or U+005E CIRCUMFLEX ACCENT `^`.
+ 
+Characters, including whitespace, are assigned meaning by the implementation.
+The definition of escapes in the `reserved-body` production, used for the body of
+a _private-use_ annotation is an affordance to implementations that 
+wish to use a syntax exactly like other functions. Specifically:
+* The characters `\`, `{`, and `}` MUST be escaped as `\\`, `\{`, and `\}` respectively
+when they appear in the body of a _private-use_ annotation. 
+* The character `|` is special: it SHOULD be escaped as `\|` in a _private-use_ annotation,
+but can appear unescaped as long as it is paired with another `|`. This is an affordance to
+allow _literals_ to appear in the private use syntax.
+
+A _private-use_ _annotation_ MAY be empty after its introducing sigil.
+
+**NOTE:** Users are cautioned that _private-use_ sequences cannot be reliably exchanged
+and can result in errors during formatting.
+It is generally a better idea to use the function registry
+to define additional formatting or annotation options.
+
+```abnf
+private-use   = private-start reserved-body
+private-start = "&" / "^"
+```
+
+> Here are some examples of what _private-use_ sequences might look like:
+> ```
+> {Here's private use with an operand: {$foo &bar}}
+> {Here's a placeholder that is entirely private-use: {&anything here}}
+> {Here's a private-use function that uses normal function syntax: {$operand ^foo option=|literal|}}
+> {The character \| has to be paired or escaped: {&private || |something between| or isolated: \| }}
+> {Stop {& "translate 'stop' as a verb" might be a translator instruction or comment }}
+> {Protect stuff in {^ph}<a>{^/ph}private use{^ph}</a>{^/ph}}
+>```
+
 #### Reserved
 
 **_Reserved_** annotations start with a reserved character
-and are intended for future standardization
-as well as private implementation use.
+and are intended for future standardization.
 A _reserved_ _annotation_ MAY be empty or contain arbitrary text.
 This allows maximum flexibility in future standardization,
-as future definitions are expected to define additional semantics and constraints
+as future definitions MAY define additional semantics and constraints
 on the contents of these _annotations_.
 A _reserved_ _annotation_ does not include trailing whitespace.
 
-Implementations MAY define their own meaning and semantics for
-_reserved_ annotations that start with
-the U+0026 AMPERSAND `&` or U+005E CIRCUMFLEX ACCENT `^` characters.
 Implementations MUST NOT assign meaning or semantics to
 an _annotation_ starting with `reserved-start`:
 these are reserved for future standardization.
@@ -433,9 +470,8 @@ While a reserved sequence is technically "well-formed",
 unrecognized reserved sequences have no meaning and MAY result in errors during formatting.
 
 ```abnf
-reserved       = ( reserved-start / private-start ) reserved-body
+reserved       = reserved-start reserved-body
 reserved-start = "!" / "@" / "#" / "%" / "*" / "<" / ">" / "/" / "?" / "~"
-private-start  = "^" / "&"
 reserved-body  = *( [s] 1*(reserved-char / reserved-escape / literal))
 reserved-char  = %x00-08        ; omit HTAB and LF
                / %x0B-0C        ; omit CR