U+1D7D9 "𝟙" Mathematical Double-Struck Digit One Unicode Character

U+1D7D9 "𝟙" Mathematical Double-Struck Digit One is a stylized numeral one rendered in a blackboard bold typeface, where the number is given a double-struck or outlined appearance, commonly used in mathematics and logic to denote specific numeric sets. It is part of the Mathematical Alphanumeric Symbols block, which was added to Unicode to provide distinct, semantically meaningful glyphs for formal notation. Unlike the standard digit "1", this double-struck version is often employed to represent the unit element in algebraic structures or the number one in contexts where typographic distinction is needed, such as in set theory or vector mathematics.

General Properties

Code Point	U+1D7D9
Version Added	3.1
Name	Mathematical Double-Struck Digit One
Block	Mathematical Alphanumeric Symbols
General Category	Decimal Number
Canonical Combining Class	Not Reordered
Bidirectional Class	European Number
Decomposition Type	Font
Decomposition Mapping	"1" U+0031 Digit One

Encodings

HTML Decimal Encoding	𝟙
HTML Hex Encoding	𝟙
UTF-8 Encoding	0xF0 0x9D 0x9F 0x99
UTF-16 Encoding	0xD835 0xDFD9
UTF-32 Encoding	0x0001D7D9
C/C++/Java Escape	\ud835\udfd9

Unicode Properties

NFC Quick Check	Yes
NFD Quick Check	Yes
Numeric Type	Decimal
Numeric Value	1
Line Break	Numeric
Changes When NFKC Casefolded	Yes
NFKC Casefold	"1" U+0031 Digit One
NFKC Simple Casefold	"1" U+0031 Digit One
Script	Common
Script Extensions	Common
Indic Syllabic Category	Other
ID Continue	Yes
XID Continue	Yes
Math	Yes
Other Math	Yes
Vertical Orientation	Rotated
Grapheme Base	Yes
Grapheme Cluster Break	Other
Word Break	Numeric
Sentence Break	Numeric