Question: I want   ![cover](https://i.stack.imgur.com/vrRNn.jpg) to denote the covering relation. I can't find this symbol in my standard sources for this sort of thing. Detextify has also failed in this. How can I get this relational symbol?

Answer: 
If you can't use `unicode-math`, here you find a couple of possibilities:

```
\documentclass{article}
\usepackage{amsmath}
\usepackage{pict2e,picture}

\newcommand{\coveringA}{%
  \mathrel{-\mkern-4mu}<%
}
\newcommand{\coveringB}{\mathrel{\text{$\vcenter{\hbox{\pictcoveringB}}$}}}

\newcommand{\pictcoveringB}{%
  \begin{picture}(1em,.5em)
  \roundcap
  \put(0,.25em){\line(1,0){.6em}}
  \put(.6em,.25em){\line(3,1){.4em}}
  \put(.6em,.25em){\line(3,-1){.4em}}
  \end{picture}%
}

\begin{document}

$x\coveringA y_{\coveringA}$

$x\coveringB y_{\coveringB}$

\end{document}

```

![enter image description here](https://i.stack.imgur.com/UZVjE.png)

If you're sure that you won't use the symbol in subscripts or superscripts, then 

```
\newcommand{\coveringB}{\mathrel{\vcenter{\hbox{\pictcoveringB}}}}

```

is more efficient. You can play with the parameters as you wish, in the second solution.
The symbol you are looking for is [U+2919](http://www.fileformat.info/info/unicode/char/2919/index.htm). You could use `unicode-math` in order to get it. It is already defined as `\mathrel` here.

```
% arara: lualatex

\documentclass{article}
\usepackage{unicode-math}

\begin{document}
\setmathfont{XITS Math}
$\righttail\lefttail$
\setmathfont{Cambria Math}
$\righttail\lefttail$
\setmathfont{Asana Math}
$\righttail\lefttail$
\end{document}

```

![enter image description here](https://i.stack.imgur.com/BN7Hg.png)

Or you choose any font you like and do:

```
% arara: lualatex

\documentclass{article}
\usepackage{fontspec}
\usepackage{mathtools}
\newcommand*{\lefttail}[1]{\mathrel{\fontspec{#1}\text{\symbol{"2919}}}}

\begin{document}
$x\lefttail{code2000.ttf} y$
$x\lefttail{quivira.otf} y$
$x\lefttail{stix-regular.otf} y$
$x\lefttail{symbola.ttf} y$
\end{document}

```

![enter image description here](https://i.stack.imgur.com/Wq3ar.png)

Some of them will need some vertical adjustment, though.

---

You may also want to take a look on [tikz-cd](/questions/tagged/tikz-cd "show questions tagged 'tikz-cd'"):

```
% arara: pdflatex

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}
\[
\begin{tikzcd}[column sep=4mm]
x & y\arrow[no head, tail]{l}
\end{tikzcd}
\]
\end{document}

```

![enter image description here](https://i.stack.imgur.com/zpQcJ.png)