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+## Avoid outdated x-prefix in comparisons as it no longer serves a purpose.
+
+### Problematic code:
+
+```sh
+[ "x$pass" = "xswordfish" ]
+
+test x"$var" = x 
+```
+
+### Correct code:
+
+```sh
+[ "$pass" = "swordfish" ] 
+
+test "$var" = ""
+```
+
+### Rationale:
+
+Some older shells would get confused if the first argument started with a dash, or consisted of `!` or `(`. As a workaround, people would prefix variables and values to be compared with `x` to ensure the left-hand side always started with an alphanumeric character.
+
+POSIX ensures [this is not necessary](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html), and all modern shells now follow suite.
+
+### Examples:
+
+Bash 1.14 from 1992 incorrectly fails this test. This was fixed for Bash 2.0 in 1996:
+
+    var='!'
+    [ "$var" = "!" ]
+
+Dash 0.5.4 from 2007 incorrectly passes this test. This was fixed for Dash 0.5.5 in 2008: 
+
+    var='('
+    [ "$var" = ")" ]
+
+Zsh (while not supported by ShellCheck) fixed the same problem in 2015.
+
+### Exceptions:
+
+This warning is optional, and not enabled by default.
+
+If you are targeting especially old shells, you should not enable this warning.
+
+### Related resources:
+
+* [Wooledge Bash Pitfall #4](https://mywiki.wooledge.org/BashPitfalls#A.5B_.24foo_.3D_.22bar.22_.5D)
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