diff --git a/spec/message.abnf b/spec/message.abnf
index b869a761fe..32cd24c942 100644
--- a/spec/message.abnf
+++ b/spec/message.abnf
@@ -10,7 +10,8 @@ variant = when 1*(s key) [s] pattern
 key = nmtoken / literal / "*"
 
 expression = "{" [s] (((literal / variable) [s annotation]) / annotation) [s] "}"
-annotation = function *(s option)
+annotation = (function *(s option)) / reserved
+
 option = name [s] "=" [s] (literal / nmtoken / variable)
 
 ; reserved keywords are always lowercase
@@ -25,7 +26,7 @@ text-char = %x0-5B         ; omit \
           / %x7E-D7FF      ; omit surrogates
           / %xE000-10FFFF
 
-literal = "|" *(literal-char / literal-escape) "|"
+literal      = "|" *(literal-char / literal-escape) "|"
 literal-char = %x0-5B         ; omit \
              / %x5D-7B        ; omit |
              / %x7D-D7FF      ; omit surrogates
@@ -34,6 +35,18 @@ literal-char = %x0-5B         ; omit \
 variable = "$" name
 function = (":" | "+" | "-") name
 
+; reserve additional sigils for future use
+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
+               / %x0E-19        ; omit SP
+               / %x21-5B        ; omit \
+               / %x5D-7A        ; omit { | }
+               / %x7E-D7FF      ; omit surrogates
+               / %xE000-10FFFF
+
 name    = name-start *name-char ; matches XML https://www.w3.org/TR/xml/#NT-Name
 nmtoken = 1*name-char           ; matches XML https://www.w3.org/TR/xml/#NT-Nmtokens
 name-start = ALPHA / "_"
@@ -44,8 +57,9 @@ name-start = ALPHA / "_"
 name-char = name-start / DIGIT / "-" / "." / %xB7
           / %x0300-036F / %x203F-2040
 
-text-escape    = backslash ( backslash / "{" / "}" )
-literal-escape = backslash ( backslash / "|" )
-backslash      = %x5C ; U+005C REVERSE SOLIDUS "\"
+text-escape     = backslash ( backslash / "{" / "}" )
+literal-escape  = backslash ( backslash / "|" )
+reserved-escape = backslash ( backslash / "{" / "|" / "}" )
+backslash       = %x5C ; U+005C REVERSE SOLIDUS "\"
 
 s = 1*( SP / HTAB / CR / LF )
diff --git a/spec/syntax.md b/spec/syntax.md
index f99feb0509..f3f552ebcc 100644
--- a/spec/syntax.md
+++ b/spec/syntax.md
@@ -7,7 +7,7 @@
    1. [Design Restrictions](#design-restrictions)
 1. [Overview & Examples](#overview--examples)
    1. [Messages](#messages)
-   1. [Expressions](#expressions)
+   1. [Expressions](#expression)
    1. [Formatting Functions](#formatting-functions)
    1. [Selection](#selection)
    1. [Local Variables](#local-variables)
@@ -19,6 +19,7 @@
    1. [Variants](#variants)
    1. [Patterns](#patterns)
    1. [Expressions](#expressions)
+       1. [Reserved Sequences](#reserved)
 1. [Tokens](#tokens)
    1. [Keywords](#keywords)
    1. [Text and Literals](#text-and-literals)
@@ -108,7 +109,7 @@ let hello = new MessageFormat('{Hello, world!}')
 hello.format()
 ```
 
-### Expressions
+### Expression
 
 An _expression_ represents a part of a message that will be determined
 during the message's formatting.
@@ -317,7 +318,7 @@ A _well-formed_ message is considered _valid_ if the following requirements are
 ### Patterns
 
 A **_pattern_** is a sequence of translatable elements.
-Patterns MUST BE delimited with `{` at the start, and `}` at the end.
+Patterns MUST be delimited with `{` at the start, and `}` at the end.
 This serves 3 purposes:
 
 - The message can be unambiguously embeddable in various container formats
@@ -345,20 +346,17 @@ Whitespace within a _pattern_ is meaningful and MUST be preserved.
 
 ### Expressions
 
-**_Expressions_** can either start with an operand or a function call.
+_Expressions_ ***must*** start with a _literal_, a _variable_, or an _annotation_. An _expression_ ***must not*** be empty.
+
+A _literal_ or _variable_ ***may*** be optionally followed by an _annotation_. 
 
-The operand is a literal or a variable name.
-The operand can be optionally followed by an _annotation_:
-a function and its named options.
-Functions do not accept any positional arguments
-other than the operand in front of them.
+An _annotation_ consists of a _function_ and its named _options_, or consists of a _reserved_ sequence.
 
-Function calls do not require an operand as an argument,
-but an _expression_ must not be completely empty.
+_Functions_ do not accept any positional arguments other than the _literal_ or _variable_ in front of them.
 
 ```abnf
 expression = "{" [s] (((literal / variable) [s annotation]) / annotation) [s] "}"
-annotation = function *(s option)
+annotation = (function *(s option)) / reserved
 option = name [s] "=" [s] (literal / nmtoken / variable)
 ```
 
@@ -398,6 +396,13 @@ Message examples:
 {{+h1 name=above-and-beyond}Above And Beyond{-h1}}
 ```
 
+#### Reserved
+
+_Reserved_ sequences start with a reserved character and are intended for future standardization. 
+A reserved sequence can be empty or contain arbitrary text. 
+A reserved sequence does not include any trailing whitespace.
+While a reserved sequence is technically "well-formed", unrecognized reserved sequences have no meaning and might result in errors during formatting.
+
 ## Tokens
 
 The grammar defines the following tokens for the purpose of the lexical analysis.
@@ -478,12 +483,12 @@ name-char = name-start / DIGIT / "-" / "." / %xB7
 
 ### Escape Sequences
 
-Escape sequences are introduced by the backslash character (`\`).
-They are allowed in translatable text as well as in literals.
+Escape sequences are introduced by the backslash character (`\`) and allow the appearance of lexically meaningful characters in the body of `text`, `literal`, or `reserved` sequences respectively:
 
 ```abnf
 text-escape    = backslash ( backslash / "{" / "}" )
 literal-escape = backslash ( backslash / "|" )
+reserve-escape = backslash ( backslash / "{" / "|" / "}" )
 backslash      = %x5C ; U+005C REVERSE SOLIDUS "\"
 ```
 
@@ -495,6 +500,7 @@ Inside _patterns_,
 whitespace is part of the translatable content and is recorded and stored verbatim.
 Whitespace is not significant outside translatable text, except where required by the syntax.
 
+
 ```abnf
 s = 1*( SP / HTAB / CR / LF )
 ```