Whereas the choice produces identical values for call and put, we seek the strike which produces absolutely identical deltas (spot, forward or driftless). This condition implies and thus
(27)
in which case the absolute delta is
. In particular, we learn, that always , i.e., there can't be a put and a call with identical values and deltas. Note that the strike is usually chosen as the middle strike when trading a straddle or a butterfly. Similarly the dual-delta-symmetric strike
can be derived from the condition .