A LEGO notebook ·
The tree of interlocks
A stud has been pressed into a tube since 1958. Since then LEGO has invented two dozen other ways for one part to hold another. Why did none of them replace the stud?
Each branch below is one way of connecting: a pin in a hole, a bar in a clip, a ball in a socket. It starts the first year Rebrickable's records show it in a set, and it hangs under the connection it grew out of. The dates are data. Which branch hangs under which is my reading.
Step 1 · The tree
Everything grows from one stud
The first bricks, in 1949, had studs on top and nothing underneath. Hollow studs and plates were in sets by 1957, the tubes came with the 1958 patent, and the stud has held ever since. Every other connection on this tree arrived after it, and nearly every one still meets a stud somewhere: a hinge between two studded plates, a Technic hole the same width as a stud, a bar that fits inside a hollow one.
1aTap a branch to trace it back to the stud. Slide the round brick to grow the tree year by year.
Even the branches that look like a clean break keep a foot on the stud. Technic's studless beam, in 1989, was meant to be joined only by pins and axles. The first one in a set had a stud connector on its end.
Twenty-four branches in seventy-seven years. But the last new one arrived in 2002.
Step 2 · It grows, then stops
New ways to connect arrive, old ones stay, and then the arrivals stop
Count the branches alive in any year and the number climbs. A new connection doesn't replace an old one: the 1977 Technic pin didn't retire the stud, and the 1999 click hinge didn't retire the 1963 hinge. Of the 24 branches that grew from the stud, 22 are still in sets today.
Then the climb stops. The last new branch, a joint that turns in clicks, came with Bionicle in 2002. LEGO has made thousands of new parts since, and every one connects in a way already on the tree.
2aEvery way to connect that was in a set that year, one plate each. Tap a year.
So the stud wasn't replaced; everything else was fitted to it. The two branches that died show what happens when a part isn't.
Step 3 · What died
Lose the ordinary brick, and the branch dies
Primo and Quatro were bricks for the smallest hands. Quatro's even took Duplo. But neither took an ordinary brick, and neither lasted: Primo eleven years, Quatro three. Duplo changed the stud's size too, but a Duplo brick takes an ordinary one, so Duplo stayed on the tree. It has been in sets every year since 1975.
LEGO also tried building systems off the grid. Modulex had studs, but on a 5 mm grid for architects' models. Scala, Znap and Clikits had none. All four are gone from LEGO's sets.
3aThe stud against the systems that changed it or left it. Tap one for its first set.
There's one connection LEGO keeps replacing on purpose. The plug.
Step 4 · The plug problem
Five power systems, five fresh starts
Motors and lights need wires, and the wires need plugs. Here LEGO has started over again and again. The 4.5 volt and 12 volt systems used small round pin plugs. 9 volt, in 1986, brought a plug shaped like a 2×2 brick. Power Functions, in 2007, brought a flat one, and Powered Up, in 2017, a six-pin one. Each new system came with new motors, new battery boxes and a plug the old ones didn't fit. Only adapter cables join them.
4aEach power system's years in sets, against the stud's. Tap one for its first set.
The contrast is the point. A plug is a connection between two pieces of electronics, and electronics move on. A stud is a connection between two pieces of plastic, fixed by a rule in 1958. The plug has started over three times. The stud has never needed to.
None of the other connections replaced the stud because each one was made to meet it. The ones that weren't are gone.
More LEGO notebooks
Sources and method
Rebrickable database, pulled Oct 24, 2026: every set except Gear and books, with minifigure parts counted in the sets they come in. A branch starts the first year any set contains one of its parts, and its studs show the share of each year's sets that contain one.
Branches are defined by Rebrickable part names: brick and plate (the stud); "Hollow Stud"; minifigure hands, minifigure tools, bars and clips (bar and clip); "Tile"; "Jumper"; brackets, "Studs on Side" and headlights; "Turntable"; "Hinge", with "Locking" for click hinges; "Towball"; "Ball Socket" and "Ball Joint"; "Wheel Holder"; gears; "Technic Brick", "Technic Axle", "Technic Pin", "Technic Beam"; pneumatics; "Rotation Joint"; "String"; "Magnet"; "Duplo Brick". Primo, Quatro, Modulex, Scala, Znap and Clikits by theme. Power systems by voltage or system name.
Which branch grows from which is an editorial reading, not a claim from LEGO.
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