[usrlit.suffix]

# 16 Library introduction [[library]](./#library)

## 16.4 Library-wide requirements [[requirements]](requirements#usrlit.suffix)

### 16.4.5 Constraints on programs [[constraints]](constraints#usrlit.suffix)

#### 16.4.5.3 Reserved names [[reserved.names]](reserved.names#usrlit.suffix)

#### 16.4.5.3.6 User-defined literal suffixes [usrlit.suffix]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L3375)

Literal suffix identifiers ([[over.literal]](over.literal "12.6 User-defined literals")) that do not start with an underscore are reserved for future standardization[.](#1.sentence-1)

Literal suffix identifiers that contain a double underscore__are reserved for use by C++ implementations[.](#1.sentence-2)