Quote from m22au:
Underlying stock is at 10.11
The following options all have the same expiry:
10.00 call (bid) $1.60
11.00 call (ask) $1.20
10.00 put (ask) $1.40
11.00 put (bid) $2.15
sell an equal number of
10.00 calls at 1.60
11.00 puts at 2.15
and buy an equal number of
11.00 calls at 1.20
10.00 puts at 1.40
You're short the 10/11 outside strangle and long the 10/11 inside strangle, short the box.
Consider the position as two identical payoffs; in this example, a short inside [synthetic] straddle and the long outside [natural] straddle. You can use the vertical or strangle arb, or use stock and consider the synthetic long and short.
Short the inside for 3.75 less the $1 strike differential, leaving 2.75. Long the natural at 2.60 leaving a credit of $.15 as stated.
The apparent edge, less commissions is the risk-premium of assignment and pinning. There is more to it, but you've omitted duration.