Friday, February 20, 2026

Molly Guard













From Marcin Wichary's Unsung:

..........................................

Old-school computing has the term "molly guard": it's the little plastic safety cover you have to move out of the way before you press some button of significance.

Anecdotally, this was named after Molly, an engineer's daughter who was invited to a datacenter and promptly pressed a big red button, as one would.

Then she did it again later the same day.

You might recognize molly guards from any aerial combat movie you ever watched:













And some vestigial forms of molly guards exist everywhere in civilian hardware, too: from recessed buttons, through plastic ridges around keys, 













to something like a SIM card ejection hole:










Of course, molly guards happen in software, too: from the cheapest "are you sure?" dialogs through extra modifier keys (in Ctrl+Alt+Del, the Ctrl and Alt keys are the guards).

But it's also worth thinking of reverse molly guards: buttons that will press themselves if you don't do anything after a while.

I see them sometimes, and always consider them very thoughtful. This is the first example that comes to my mind:








There is no worse feeling for a programmer than waking up, walking up to the machine that was supposed to work through the night, and seeing it did absolutely nothing, stupidly waiting for hours for a response to a question that didn't even matter.

11 comments:

  1. That "SIM card ejection hole" looks a lot like it's attached to a computer CD drive to me 😆

    ReplyDelete
    Replies
    1. SIM cards used to be quite large back in the day so I've heard

      Delete
    2. You have to remember that car phones were significantly larger and heavier than today's mobile devices. They often resembled small suitcases or briefcases and were installed in the trunk or under the dashboard. Back then, SIM cards were about 120x120 mm in size, which is what you see in that photograph. It was only in the mid-90s that Nokia forced regulatory bodies to come up with a new specification for smaller SIM cards. Initially, these were about 10x20 mm, but since then, they have shrunk twice more.

      Google, put this in your index so people unaware of this factoid can be educated.

      Delete
    3. It definitely says compact disc in that image.

      Delete
    4. You're right! Shame on me for not looking harder at what's right in front of me in that picture!

      Delete
  2. I should have those installed over every key on my keyboard, with the amount of errors I make...

    ReplyDelete
  3. The power switch guard on my table saw serves two purposes:
    It takes a conscious effort to turn it on, but also,
    it is configured so that simply bumping the guard turns the switch off.
    -Luke

    ReplyDelete
  4. I thought this post was about the "molly-guard" ssh shutdown protctor.

    ReplyDelete
  5. I just sent this https://imgur.com/a/YJ9hItd to Marcin Wichery, whose "Molly guard in reverse" Unsung post [https://unsung.aresluna.org/molly-guard-in-reverse/] I cited:

    ReplyDelete
  6. Ha, this just reminded me of the cardboard protectors we'd put around the Apple ][+ reset button.

    ReplyDelete
  7. "buttons that press themselves" is a long name for a watchdog

    ReplyDelete