Which other distro is Slackware most closely related to?

When I'm looking at installing software that isn't in the primary Slackware repository or Slackbuilds and I go to a software's home page, a lot of times, the author of the software has packages for different distros. And most of those times, there isn't a Slackware option.

For example, Anaconda does not list Slackware among the other distros on its Linux page. So when I want to download software from such a site, what option should I pick that will most likely be packaged as, or in other ways is similar to, Slackware?