平面

What is a Unicode Plane?

A plane is a group of 65,536 contiguous code points in the Unicode code space. Unicode divides its 1,114,112 total code points into 17 planes, numbered 0 through 16. Each plane spans exactly 0x10000 (65,536) code points:

• Plane 0: U+0000–U+FFFF
• Plane 1: U+10000–U+1FFFF
• Plane 2: U+20000–U+2FFFF
• ...
• Plane 16: U+100000–U+10FFFF

The plane number appears as the first two hex digits of a 6-digit code point. For example, U+1F600 is in Plane 1 (the leading 1), and U+20001 is in Plane 2.

The 17 Planes

Why Planes Exist

The plane structure was introduced when Unicode expanded from a 16-bit code space (65,536 points) to a 21-bit code space (1,114,112 points). The 16-bit expansion was called UCS-2; the 21-bit extension was encoded as UTF-16 using surrogate pairs for code points above the BMP.

The upper bound of U+10FFFF was chosen deliberately to match the maximum value encodable by UTF-16 surrogate pairs (which can reach exactly 1,048,576 supplementary characters, plus the 65,536 BMP positions = 1,114,112 total). This made UTF-16 the natural boundary for the code space.

Accessing Code Points in Planes

# Python: code point value reveals the plane
cp = 0x1F600  # 😀 GRINNING FACE
plane = cp >> 16   # Right-shift 16 bits to get plane number
print(plane)       # 1 (Plane 1 / SMP)

# Check if BMP
is_bmp = cp <= 0xFFFF  # False for 😀

// JavaScript: detect supplementary plane characters
function isSupplementary(cp) {
  return cp > 0xFFFF;
}
console.log(isSupplementary(0x1F600)); // true

Planes in Encoding

The plane determines how a character is encoded in variable-length encodings:

Planes 4–13 are entirely unassigned, creating a large reserved area for future expansion.

Common Misconceptions

"Planes are always full" — Most planes are sparsely populated. Plane 1 has many assigned characters but also large unassigned regions. Planes 4–13 are completely empty.

"Higher plane = less important" — Plane 1 contains all modern emoji and many important historic scripts. Plane number reflects historical encoding order, not character importance.

Quick Facts