The BWO Parachute is the standard adult Blue-Winged Olive imitation. The hackle is wound horizontally around a single upright post instead of around the hook shank, so the fly sits flush in the surface film with the body kissing the water - the posture of a real BWO dun drying its wings. That low ride and the visible post make it both convincing to trout and easy for you to track on the drift.
Tie it olive or light olive, sizes 16-22. Match the smaller end (20-22) on tailwaters and spring creeks where Baetis run tiny; 16-18 covers bigger freestone bugs.
Fish it during a BWO emergence, which fires hardest on cool, overcast, drizzly days - water in the mid-40s to upper-50s is prime. Look for trout sipping in soft seams and the tails of pools, then put a dead-drift over them with a downstream reach so the fly reaches the fish before the leader. If they refuse the dun, drop a BWO emerger off the bend - the parachute then carries the emerger to the fish without a separate indicator.