C20 or C-20 may refer to:
Science and technology
- Carbon-20 (C-20 or C), an isotope of carbon
- C20, the smallest possible fullerene (a carbon molecule)
- C20 (engineering), a mix of concrete that has a compressive strength of 20 newtons per square millimeter
- Malignant neoplasm of rectum, a type of colorectal cancer (ICD-10 code: C20)
- Caldwell 20 or North America Nebula, an emission nebula in the constellation Cygnus
- IEC 60320 C20, a polarised, three pole socket electrical connector
- C20 (Capacity, 20 hours), C100 (100 hours), battery capacity
Transportation and military
- Colt Canada C20 DMR, a Canadian designated marksman rifle
- HMS C20, a 1910 British C-class submarine
- Sauber C20, a 2001 racing car
- SL C20, a type of rolling stock used in the Stockholm metro
- HMS Tiger (C20), a cruiser of the Royal Navy
- C-20, the proposed in-service designation of the Fokker F-32
- C-20A/B/C/D/E, variants of the Gulfstream III
- C-20F/G/H/J, variants of the Gulfstream IV
- C20 is a version of the Allison Model 250 aircraft engine
Other uses
- Bill C-20, the Canadian Clarity Act
- OMX Copenhagen 20, Denmark's primary stock market index
- King's Pawn Game, a chess opening (Encyclopaedia of Chess Openings code: C20)
- The Twentieth Century Society, a British charity which campaigns for the preservation of architectural heritage from 1914 onwards
- C20 Cities, now expanded to the C40 Cities Climate Leadership Group
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