S21

S21 or S 21 stands for:

S21 (Berlin) , S-Bahn construction project in Berlin
S21 (Hamburg) , S-Bahn line in Hamburg
S21 (Carinthia) , S-Bahn line in Carinthia
Stuttgart 21 , rail transport and urban development project
State road 21 in Saxony, see list of state roads in Saxony up to the S 199 # StSn21
Dataphon S21 , acoustic coupler

S ₂₁ stands for:

the forward transmission factor S ₂₁ for the scattering parameters, see scattering parameter #Zweitore

S-21 stands for:

S-21 (Torture Prison ) , Khmer Rouge Torture Prison, see Tuol Sleng Genocide Museum

S-21: The Khmer Rouge's death machine , feature film about the torture prison S-21

Letov Š-21 , Czechoslovak aircraft

De Schelde S-21 , Dutch fighter plane

Sukhoi S-21 , Russian discontinued aircraft project of a supersonic business jet