What is 空白字符?

表示水平或垂直空间但无可见字形的字符，Unicode定义了17种以上具有不同宽度和换行行为的空白字符。

用于分隔句子部分或表示范围的标点，Unicode定义了多种破折号：连字符（‐）、半角破折号（–）、全角破折号（—）、数字破折号（‒）等。

排版印刷

全角 / 半角（排版单位）

全角（Em）：等于字号的宽度；半角（En）：全角的一半，用于定义全角破折号宽度、全角空格、半角空格和CSS单位（1em、0.5em）。

2023-05-29 · Updated 2024-11-05

What are Em and En?

The em and en are typographic units of measurement that describe relative sizes and spacings within a font. Unlike absolute units (pixels, points, millimeters), the em and en are proportional — they scale with the font size, making them fundamental to responsive and harmonious typography.

An em is equal to the current font size. If you're setting text at 16px, 1 em = 16px. If the font size is 24pt, 1 em = 24pt. The name comes from the width of the uppercase letter "M," which was traditionally a square in metal type (meaning it was as wide as it was tall — one em square). Modern typefaces don't always make "M" exactly one em wide, but the unit retains the name.

An en is exactly half an em. Historically, the letter "N" was half the width of "M," hence the name.

Em and En in Typography

These units govern the width of key typographic elements:

Element	Width	Unicode
Em dash	1 em	U+2014 —
En dash	1 en (½ em)	U+2013 –
Em space	1 em	U+2003
En space	1 en	U+2002
Thin space	⅕ em	U+2009
Hair space	1/24 em (approx)	U+200A
Figure space	Width of a digit	U+2007

The em dash is named because it is (approximately) one em wide. The en dash is named because it is one en wide. This relationship holds even as font sizes change — at 8pt, an em dash is narrower than at 36pt, but it always fits within its font's em square.

Em in CSS

In CSS, the em unit is relative to the current element's font size:

h1 {
  font-size: 2rem;    /* 2× root em (absolute) */
  margin-bottom: 0.5em; /* 0.5 × h1's own font-size */
  padding: 0.25em 0.5em; /* relative to h1's font-size */
}

This creates a critical distinction from rem (root em), which is always relative to the <html> element's font size:

/* If html { font-size: 16px; } */
.parent { font-size: 20px; }
.child {
  font-size: 1em;  /* = 20px (parent's size) */
  font-size: 1rem; /* = 16px (root size) */
}

Em in Other Contexts

CSS text-indent and letter-spacing: using em ensures the indent or spacing scales proportionally with font size changes.

Metal type: The em was a physical unit — the square body of a type block. Compositors used em quads (full em) and en quads (half em) as blank spacing pieces inserted between words and at line ends. This physical heritage is why the em is so fundamental: it was the basic unit of the typesetter's trade.

Print design: InDesign and Illustrator display em values in many spacing controls. An indent of 1 em in a 12pt paragraph = 12 points of indent.

Quick Facts

Property	Value
1 em	Equals current font size
1 en	Equals ½ em
Origin of "em"	Width of capital M in metal type
Origin of "en"	Width of capital N (≈ half M) in metal type
Em dash Unicode	U+2014
En dash Unicode	U+2013
Em space Unicode	U+2003
CSS em unit	Relative to current element's font-size
CSS rem unit	Relative to root element's font-size