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A Lego clone is a line or brand of children's construction blocks which is mechanically compatible with Lego brand blocks, but is produced by another manufacturer. The blocks were originally patented by The Lego Group in 1961 as "toy building bricks", and the company has since remained dominant in this market. Some competitors have moved to take advantage of Lego brand recognition by advertising their own products as compatible with Lego, with statements such as "compatible with leading building bricks".
While the legal status of providing compatibility with Lego brand blocks is unclear, the underlying patents of invention are long expired, opening the field to rivals. Shortages of Lego's own branded product, such as after the release of 2014's The Lego Movie, have encouraged independent retailers to stock compatible rival products.
At least two of the largest clone manufacturers have been challenged in court by Lego. The lawsuits have been mostly unsuccessful, for courts have generally found the functional design of the basic brick to be a matter of patent rather than trademark law, and all relevant Lego patents have expired.
The Canadian company Mega Bloks was sued on the grounds that its use of the "studs and tubes" interlocking brick system was a violation of trademarks held by Lego. On November 17, 2005, the Supreme Court of Canada upheld Mega Bloks' right to continue selling the product in Canada. A similar decision was reached by the European Union's Court of First Instance on November 12, 2008, upholding an EU regulatory agency's reversal of opinion following an objection by Mega Bloks against a trademark awarded to Lego in 1999. On September 14, 2010, the European Court of Justice ruled that the 8-peg design of the original Lego brick "merely performs a technical function [and] cannot be registered as a trademark." 
The English company Best-Lock Construction Toys sued Lego in German courts in 2004  and 2009. The German Federal Court denied Lego trademark protection for the shape of its bricks in the latter case.
The Lego Group did score a success in 2002, when its Swiss subsidiary Interlego AG sued the Tianjin CoCo Toy Co., Ltd. company for copyright infringement. A claims court found many CoCo bricks to be infringing; CoCo was ordered to cease manufacture of the infringing bricks, publish a formal apology in the Beijing Daily, and pay a small fee in damages to Interlego. On appeal, the Beijing High People's Court upheld the trial court's ruling.
Major clone manufacturers
- BanBao - based in China. Recently won a lawsuit brought by the Lego Group, due to differentiation of its commercial offerings from the Lego Group's.
- Best-Lock - based in England. Compatible with Lego bricks.
- Brictek - based in Quebec, Canada. Founded 2012, compatible with Lego bricks. Offers various sets, such as trains, at approximately 3/4 the cost of the name-brand product.
- Built to Rule - a compatible line from Hasbro based upon existing toys and characters from the Hasbro brand, such as G.I. Joe and Transformers.
- Cobi - based in Poland. Continues to be marketed independently since merging with Best-Lock in 2006.
- Character Options - based in the UK. They produce products of compatible bricks in several ranges including Doctor Who, HM Armed Forces, Deadly 60, Ben 10, and others.
- CoCo - based in China. Produced by Tianjin COKO Toy Co., Ltd. until 2002.
- Cogo - based in the Far East.[where?] Produced by Shantou Little White Dragon Toy Industry Co., Ltd./Loongon Toy Industry Co., Ltd. Appear similar to Lego bricks and Lego sets.
- Enlighten Brick - based in China. Produced by Concord Toys International Ltd.
- K'nex - based in the US. Large engineering-themed toy producer, mainly non compatible girder-based elements, but some sets feature Lego-compatible studs. K'nex bricks have holes in them.
- Kre-O - produced by Hasbro, and released six years after the end of the "Built to Rule" line. Sets come with Minifigure-like characters called Kreons.
- Laser Pegs- based in the US. They are "The Original Lighted Construction Set" with the main distinguishing factor being that it lights up.
- Lite Brix - based in the US. They are in patent a dispute with Laser Pegs over similar technology.
- Mega Bloks - based in Canada. The Lego Group's closest competitor, with a large product line, major third-party licenses such as World of Warcraft, Need for Speed, Halo, Call of Duty, Smurfs, Skylanders, Barbie and Hello Kitty, and a presence in mainstream toy outlets.
- Nintendo - based in Japan. Manufactured Nintendo Block (later rebranded N&B Block) starting in 1968, notable for featuring curved blocks before Lego offered them in Japan. The blocks were compatible with Lego. N&B ceased production in the early 1970s.
- Oxford - based in South Korea. Distributed in the West by Hasbro under the Kre-O brand.
- Rokenbok - based in the US.
- Sluban - based in China. Their sets are largely clones of Oxford sets.
- Star Diamond - based in China. Their sets are clones of Lego sets.
- Tyco Super Blocks - based in the US.
- Super Blox - based in the US. A brand of the CRA-Z-ART company.
- US patent 3005282, Christiansen, Godtfred Kirk, "Toy Building Brick", issued 1961-10-24, assigned to Interlego A.G.
- "Lego shortage leaves independent stores with empty shelves". CBC News. 15 December 2014.
- Mega Bloks' Appeal to the Supreme Court of Canada via CanLII.org
- "Lego loses trademark ruling in EU". nytimes.com. October 12, 2008
- Montreal Gazette summary of euro court case decision[dead link]
- von RA Dennis Breuer (2012-04-19). "Pressemitteilung des BGH Nr. 158/2009: Legostein als Marke gelöscht | markenmagazin:recht". Markenmagazin.de. Retrieved 2012-10-09.
-  Judgement of the German Federal Court (press release 147/2004)
- "News". Ccpit-patent.com.cn. Retrieved 2012-10-09.
- "Canadian company Brictek thrives amid Lego shortage". Canadian Broadcasting Corporation. 16 December 2014.