We compute, mimicking the Lascoux-Schützenberger type A combinatorial procedure, left and right keys for a Kashiwara-Nakashima tableau in type C. These symplectic keys have a similar role as the keys for semistandard Young tableaux. More precisely, our symplectic key gives a tableau criterion for the Bruhat order of the hyperoctahedral group and describes the type C Demazure atoms and characters. The right and the left symplectic keys are related through the Lusztig involution.
