On Brothers Brick, there is an AFOL term for bad LEGO combos: Illegal. These combinations are "unconventional" and put stress on the bricks. Essentially, they are nonconformist, risky maneuvers, typically compromising the integrity of any parts in it.

For example -- from the site -- wedging a flat plate between studs on a brick. I have used this term before, and can testify to dangerous combinations. So I'm posing this as another challenge for the community, like my hard to separate combos.

What are some Illegal combinations * you* have found? Largest number of unique combos wins! (please don't try to kill anything for this.)

  • I would again politely ask the community to like the question being answered as general policy, after all, no good questions means no good answers!
    – user6907
    May 5, 2016 at 22:56
  • How can LEGO building techniques be illegal? Would you be put In jail for using on? I like to wonder
    – Estelle
    Feb 13, 2018 at 9:42
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    "Illegal" is just the word lego designers use to define building techniques that are considered undesirable because: - they place stress on te elements, making them wear out - they cause elements to be out of sync, making them difficult to combine with standard building techniques or difficult to model with designer software - they make the set difficult to assemble or dissassemble for a kid - ... There are of course no repercussions if you use these techniques anyway, it is your LEGO... Feb 13, 2018 at 10:28

2 Answers 2


I do not think it is up to us to determine what is considered an Illegal Technique - TLG (The LEGO Group) determines this for their commercial products. It depends on how much of a purist you are whether you apply these constraints in your own builds. That said, LEGO Designer Jamie Berard has made a presentation in the past that highlights most Illegal Techniques.

Some examples:

  • Studs-on-side bricks cannot have their side stud in the hole of a technic brick
  • Bricks mounted SNOT style onto Ehrling/Headlight bricks cannot sit directly above a normal stud (the minimal height of the LEGO embossing makes that combination illegal)

  • Technic pins have to be fully inserted into technic holes, otherwise they stay in compressed state which causes stress on the element.

  • Plates cannot be inserted with their studs into technic bricks, unless it is a 1x1 plate (so only 1 hole is used) and there is no brick attached to the technic brick directly above the inserted plate.

  • As mentioned by you already, you cannot insert plates between studs (this used to be a valid technique 20+ years ago, but is no longer considered "legal" by LEGO).

  • LEGO designers are also strongly discouraged from ever directly mating polycarbonate or transparent pieces. I don't know if this counts as "illegal" or if there is any official set that breaks this rule.
    – user3971
    May 5, 2016 at 15:20
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    @jdv certain trans pieces cannot be used with other trans pieces because the plastic forms a chemical bond between the elements. The linked PDF gives the example of a transparent bar inserted into another transparent element such as a cone. Stud-to-stud connections are still allowed. May 8, 2016 at 4:16
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    Is putting a round piece between four studs legal?
    – Sandra
    Feb 22, 2020 at 7:59

One illegal technique I've seen was two elements stuck together using a sticker to bind them. The builder claimed that the solution was "purist" because it was a lego sticker from a set.

  • This is not even a building technique. :)
    – Metalbeard
    Mar 5, 2019 at 20:54
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    @Aziraphale It was used in a moc to hold pieces together... depends on your point of view I guess. Load-bearing stickers. Mar 5, 2019 at 21:06