Plan supplémentaire

What are Supplementary Planes?

Supplementary planes are the 16 Unicode planes numbered 1 through 16 — all planes beyond Plane 0 (the Basic Multilingual Plane). They cover code points U+10000 through U+10FFFF, and they contain characters that did not fit in the original 16-bit BMP design: historic writing systems, rare CJK ideographs, all modern emoji, musical notation, mathematical symbols, and large private use areas.

The term "supplementary character" refers to any character with a code point in these planes. Supplementary characters require special handling in UTF-16 (surrogate pairs) and use 4 bytes in UTF-8.

The Supplementary Planes

Plane 1: Where Emoji Live

The Supplementary Multilingual Plane (SMP) is the most practically important supplementary plane. It contains:

• Emoji (U+1F300–U+1FAFF): All major emoji ranges
• Historic scripts: Linear B (first European writing system), Gothic, Ugaritic, Cuneiform, Egyptian Hieroglyphs, Old Persian
• Musical notation: U+1D100–U+1D1FF
• Mathematical Alphanumeric Symbols: U+1D400–U+1D7FF (𝒜, 𝔹, ℂ...)
• Playing Cards, Dominos, Mahjong: Miscellaneous symbols

Plane 2: The CJK Overflow

The Supplementary Ideographic Plane (SIP) was created specifically because the ~20,000 CJK ideographs in the BMP were insufficient for full CJK coverage. Plane 2 adds extensions B through F, bringing the total CJK ideograph count above 90,000. These characters are primarily needed for rare classical Chinese texts, historical documents, and personal name kanji not in common use.

Encoding Supplementary Characters

UTF-16 Surrogate Pairs

Because UTF-16 code units are 16-bit and supplementary code points exceed 16-bit range, UTF-16 encodes them as two 16-bit surrogate code units:

Encoding U+1F600 (😀) in UTF-16:
  Step 1: Subtract 0x10000:  0x1F600 - 0x10000 = 0xF600
  Step 2: High 10 bits: 0x3D → add 0xD800 = 0xD83D (high surrogate)
  Step 3: Low 10 bits: 0x200 → add 0xDC00 = 0xDE00 (low surrogate)
  Result: 0xD83D 0xDE00

UTF-8

UTF-8 uses 4 bytes for supplementary characters:

U+1F600 → 0xF0 0x9F 0x98 0x80

UTF-32

UTF-32 stores every code point in 4 bytes — no special handling needed for supplementary characters.

Impact on Programming

# Python 3: str handles all Unicode natively
s = "😀"  # U+1F600, Plane 1
len(s)     # 1 — correct

# ord() and chr() work for all code points
print(ord("😀"))         # 128512
print(chr(128512))       # 😀
print(f"U+{ord('😀'):06X}")  # U+01F600

// JavaScript must use surrogate-aware methods
const emoji = "😀";
emoji.length;              // 2 (UTF-16 surrogate pair!)
emoji.codePointAt(0);     // 128512 (correct code point)
String.fromCodePoint(128512); // "😀"
[...emoji].length;         // 1 (spread uses code points)

Quick Facts