# \atop

Plain-TeX infix generalized fraction without a fraction bar, placing the left formula over the right formula.

https://texform.dev/latex/commands/atop

## Metadata

| Field | Value |
| --- | --- |
| Topic | [Fractions and roots](https://texform.dev/latex/topics/fractions-roots) |
| Packages | base |
| Allowed mode | math |
| Tags | discouraged |
| Commonality | low |
| Kind | infix |

## Syntax

### base

Argument specification: ``

Plain-TeX infix generalized fraction without a fraction bar, placing the left formula over the right formula.

## Examples

```latex
a \atop b
```

[Open in Playground](https://play.texform.dev/?input=a+%5Catop+b)

## Renderer support

| Package | MathJax | KaTeX | XeTeX |
| --- | --- | --- | --- |
| base | full | full | full |

## Usage frequency

0.11% of formulas in the measured corpus contain this entry, rank 105 of 263 entries with frequency data.

## In TeXForm

TeXForm parses this entry as infix syntax; the package determines its argument specification.

### Discouraged in base

This is a plain-TeX infix generalized fraction primitive. LaTeX documents normally prefer brace-delimited forms such as \genfrac.

```latex
$1 \atop $2 -> \genfrac{}{}{0pt}{}{$1}{$2}
```

\genfrac is the standard LaTeX form for barless generalized fractions and avoids the ambiguous grouping scope of the infix primitive.

### [base/atop-to-genfrac](https://texform.dev/rules/base/atop-to-genfrac)

Rewrite infix atop to an explicit no-rule genfrac form.

Level: authoring. Fidelity: render.

```latex
#1 \atop #2
```

→

```latex
\genfrac{}{}{0pt}{}{#1}{#2}
```

```latex
x_1+\cdots+x_m \atop y_1y_2\cdots y_n
```

→

```latex
\genfrac{}{}{0pt}{}{x_1+\cdots+x_m}{y_1y_2\cdots y_n}
```

## Equivalent forms

```latex
\genfrac{}{}{0pt}{}{$1}{$2}
```

exact. 

## See also

- [\\above](https://texform.dev/latex/commands/above)
- [\\abovewithdelims](https://texform.dev/latex/commands/abovewithdelims)
- [\\atopwithdelims](https://texform.dev/latex/commands/atopwithdelims)
- [\\brace](https://texform.dev/latex/commands/brace)
- [\\brack](https://texform.dev/latex/commands/brack)
- [\\choose](https://texform.dev/latex/commands/choose)
- [\\over](https://texform.dev/latex/commands/over)
- [\\overwithdelims](https://texform.dev/latex/commands/overwithdelims)

## Test cases

### base

| Input | Case | MathJax | KaTeX | XeTeX |
| --- | --- | --- | --- | --- |
| a \atop b | Positive: baseline | Pass | Pass | Pass |