I'm looking for a word for the thingy that's in the blue circle - ideally something not too complicated for a 3-year-old buddy. :)


  • 3
    I would call this part of a ball and socket joint.
    – Zairja
    Commented Oct 12, 2012 at 12:39
  • protruberance ?
    – mgb
    Commented Oct 12, 2012 at 12:42
  • FTR note that on a jig-saw, it is often/usually called a "tab" - since the industries are similar, it's quite possible in the industry in question it would be called a "tab".
    – Fattie
    Commented May 31, 2015 at 4:22
  • 4
    it definitely is not a ball and socket joint. A ball and socket joint is a particular type of coupling in engineering of the various couplings which allow movement. (Such as a hinge, pinion, universal joint, ball and socket, etc etc.) The item in question is a type of "joint" in the woodworking sense of a fixed joint (like a dovetail, mortise, whatever).
    – Fattie
    Commented May 31, 2015 at 4:25
  • 1
    This is called a wooden ball connector
    – user194757
    Commented Sep 4, 2016 at 20:21

2 Answers 2


I like the word connector. It's generic enough that it would work for the ball or the socket, and also for the male and female ends of the other track pieces.

I don't think connector is too advanced for a 3-year-old. A connector connects two things together. If the child doesn't already know its meaning, it's an age-appropriate vocab word.

I might wait awhile, though, before using male and female to describe the other joints.

  • 1
    It may go without saying, but I would add that all the answers provided for the OP should probably be used at least once. I'm pretty sure it's a good thing to promote a rich vocabulary, so long as the kid isn't overloaded. Some words may be beyond his ken (and pronunciation) at that age.
    – Zairja
    Commented Oct 12, 2012 at 13:12
  • 1
    "Connector" is what we called it when I taught Sunday School. My students would get confused when I called it a "joint" because they either had a pre-existing mental association of that word with drugs, or because to them "joint" meant something like "elbow" -- that bends. (Should mention that I taught Sunday school in a not-so-nice part of town, hence the drug terminology known to 5 year olds.) Commented Oct 12, 2012 at 14:12
  • I agree with @Zairja: introduce both words. Generically it's a connector; specifically a knob connector that fits in the slot connector.
    – ghoppe
    Commented Oct 12, 2012 at 18:00

This is known as a "ball joint". They're often seen in puppets, armatures, and other toys, as well as in the human body.

Edit: Just saw the updated question. tchrist's suggestion of knob works great, and in describing how it works, you can simply say it plugs in or connects another piece. Although I think there's nothing wrong with exposing a 3-year-old kid to "advanced" terminology like joint.

ball joint

Pictured: a LEGO Bionicle Ball Joint

Edit #2: Jon Hanna made a good point in the comments below. Upon closer inspection, it may be more accurate to describe the track as using a "pivot joint" (#5 in the picture below). I mistakenly assumed that the ball would plug into the next section of track rather than lay in the cut out groove.


I won't be miffed if another answer is now accepted instead as I think they're both excellent alternatives!

  • 6
    It's not a ball-joint though, since it isn't used to allow rotation through different planes as a ball-joint does, but rather to hold the two pieces of track in a single place. It's hence a knob.
    – Jon Hanna
    Commented Oct 12, 2012 at 13:39
  • @JonHanna I can't verify it, since I don't own the train track in question, but I would assume that if the tracks weren't laid on the ground that they would be able to swivel. Keep in mind a ball joint doesn't need a full range of motion to be called as such (e.g. human hips). Regardless, I had already upvoted the other answers, which I think are just as good (and said as much in my previous edit and comments), though I stand by my answer and appreciate your clarification. :)
    – Zairja
    Commented Oct 12, 2012 at 14:04
  • 3
    The track doesn't move. It can do though if you lift it up, so it does act as a ball-joint in that condition, though it isn't used for it. In that case, it is a specific sort of ball-joint - an enarthrosis - which is to say a ball-and-socket joint where the socket covers more than half of the ball. I shall resist offering enarthrosis as my answer though, as cool a word as it is :)
    – Jon Hanna
    Commented Oct 12, 2012 at 14:45
  • That is an excellent word! Looking closer at the picture, it appears that the ball rests within a groove of the connecting track, in which case it may be most accurate to call it a pivot joint (see #5 of the diagram). It could pivot were it not on the floor. I'm updating my answer with this information.
    – Zairja
    Commented Oct 12, 2012 at 14:49
  • 2
    I wonder if "joint" is the right term. It doesn't fit the mechanics definition "a connection between two rigid bodies which allows movement with one or more degrees of freedom between them" (wikipedia). Not sure about this, interested in more discussion.
    – David W
    Commented Oct 12, 2012 at 17:06

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