> For the complete documentation index, see [llms.txt](https://overleaf-pro.ayaka.space/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://overleaf-pro.ayaka.space/latex/zh-tw/latex-cuo-wu/04-double-subscript.md).

# 雙下標

## 簡介

按照慣例，LaTeX 使用底線字元（`_`）來排版下標，例如 `$a_b$` 會產生 $$a\_b$$。如果你寫下 `$a_bcde$` 只有第一個字元 `b`，是下標， $$a\_bcde$$，因為 LaTeX 不會 *預設* 你想把整組字元都設為下標 `bcde`。若要將多個項目（token）排成下標，你需要透過將它們括在一個群組中來告訴 LaTeX `{...}`，例如： `$a_{bcde}$`，會產生 $$a\_{bcde}$$.

## 雙重下標錯誤

當 LaTeX 被要求為某段已經 *已經* 帶有下標的數學式再加上下標時，就會出現雙重下標錯誤。這個錯誤通常可透過使用大括號來解決 `{...}` ，它在數學模式中會產生所謂的 *子公式*——用來描述你嘗試排版的數學表達式片段的術語。

例如，寫成 `$a_b_c$` 會產生以下雙重下標錯誤：

![Overleaf 上顯示的雙重下標錯誤](/files/10f77498f8c0ada34f6a66b15667382fee2df407)

這個特定錯誤可透過使用大括號以多種方式修正 `{...}`——結果會依大括號的位置而有所不同：

* `$a_{b_c}$` 排版為 $$\displaystyle a\_{b\_c}$$
* `${a_b}_c$` 排版為 $${a\_b}\_c$$
* `$a_{bc}$` 排版為 $$a\_{bc}$$

### 雙重下標錯誤：特殊情況

一個關於 `tex.stackexchange` 突顯了 [雙重下標錯誤的一個令人意外的例子](https://tex.stackexchange.com/questions/253080/why-am-i-getting-a-double-subscript-error)，其成因與對應的修正方式或許值得了解。我們在此重現這個例子。

即使使用了大括號，以下範例仍無法編譯：

```latex
\documentclass{article}
\begin{document}
\({\vec a_b}_c\)
\end{document}
```

[在 Overleaf 中開啟這個 ***會產生錯誤的*** 範例](https://www.overleaf.com/docs?engine=pdflatex\&snip_name=accents+package+example\&snip=%5Cdocumentclass%7Barticle%7D%0A%5Cbegin%7Bdocument%7D%0A%5C%28%7B%5Cvec+a_b%7D_c%5C%29%0A%5Cend%7Bdocument%7D)

修正此問題的一種方法是加入 [`accents` 套件](https://ctan.org/pkg/accents?lang=en) 到文件前言中（如同 [此處](https://tex.stackexchange.com/a/253094)):

```latex
\documentclass{article}
\usepackage{accents}
\begin{document}
\({\vec a_b}_c\)
\end{document}
```

[在 Overleaf 中開啟這個範例](https://www.overleaf.com/docs?engine=pdflatex\&snip_name=accents+package+example\&snip=%5Cdocumentclass%7Barticle%7D%0A%5Cusepackage%7Baccents%7D%0A%5Cbegin%7Bdocument%7D%0A%5C%28%7B%5Cvec+a_b%7D_c%5C%29%0A%5Cend%7Bdocument%7D)

## 更多關於下標的內容

可以透過撰寫類似以下的 LaTeX 來排版更深層的下標： `$a_{b_{c_{d_e}}}$` 其排版結果為 $$a\_{b\_{c\_{d\_e}}}$$。第二層或更低層級的所有下標（次下標）都會以相同的字型大小（以點為單位）排版，而第一層下標則會以稍大的字型大小（點數）排版。若要進一步了解下標，請造訪 Overleaf 的 [專門介紹此主題的說明頁面](/latex/zh-tw/shu-xue/02-subscripts-and-superscripts.md).


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