Template:\sandbox
Returns "/sandbox" – or, if supplied, the first unnamed parameter ("return this" above) – if the template detects that the page where it is transcluded is a subpage named "sandbox" (i.e. a sandbox page); otherwise it returns nothingTemplate:Spndashor, if supplied, the second unnamed parameter ("else return this" above). It corresponds to {{When pagename is |/sandbox={{{return this|/sandbox}}} |other={{{else return this|}}}}}.
This can be useful to e.g. switch the version of a template used in its /doc[umentation] to the version in its sandbox (by replacing {{template_name ... on the /doc page with {{template_name{{\sandbox}} ...) when that documentation is transcluded on the template's sandbox page.