Put call parity

(post from old site)

C = max{S-K,0}
P = max{0,K-S}


C + Kd(0,T) = S + P
@maturity 
If S > K,
S-K + K = S + 0
If K > S,
0 + K = S + K-S

Graphically,
lhs is call curve shift up by K
Rhs is stock curve (45 degree) with left tail shifted up and flattened.