The `\not`

symbol typesets a zero width character which protrudes on the right and so it is apt to be superimposed only on certain characters having a suitable width (the equal sign, for example); since the two arrows are wider than the equal sign, the alignment is not optimal. A solution is to push the `\not`

a bit on the right and backspacing for the same amount:

```
\newcommand\nleftrightarrow{
\mathrel{\mkern2.1mu\not\mkern-2.1mu\leftrightarrow}}
\newcommand\nLeftrightarrow{
\mathrel{\mkern2.1mu\not\mkern-2.1mu\Leftrightarrow}}
```

You might also consider the set of "negated arrows" provided by **amssymb**; the names are the same that I chose:

```
\usepackage{amssymb}
$X\nleftrightarrow Y$ and $X\nLeftrightarrow Y$
```

Try both and choose what suits your taste. If you already load **amssymb** use `\renewcommand`

instead of `\newcommand`

.

One can also define a `\xnot`

command that gives to the symbol the width of the equal sign and use a standard method for superimposing symbols:

```
\newcommand\xnot{\mathpalette\xxnot}
\newcommand*\xxnot[2]{\mathrel{\ooalign{%
\hfil$#1\not\mathrel{\hphantom=}$\hfil\cr\hfil$#1#2$\hfil\cr}}}
```

Now `\xnot\leftrightarrow`

gives the same result as the "definition by hand" before. The strange `\mathrel`

after `\not`

has its explanation in the fact that TeX never adds space between two relation math atoms. Thanks to Martin for having suggested this method. The **centernot** package does something similar.

For this problem the solution is quite simple:

```
\mathrel{+}=
```

so that the combination will be treated as a relation, since `=`

is a relation and TeX never puts space between two relation symbols. So

```
$a \mathrel{+}= b$
```

will be rendered as

You might want to define a shorthand

```
\newcommand{\pluseq}{\mathrel{+}=}
```

and input the formula above as

```
$a \pluseq b$
```

## Best Answer

You could use

`\left...\right`

for stretchable delimiters, or perhaps one of the pairs of the`\bigl...\bigr`

family of commands: