Double your chances with a dry dropper rig: dry fly on the surface that also serves as a strike indicator followed by a emerger or nymph

Dave Rothrock's The Ranting Angler

Fly Fishing Rigs: The Dry-Dropper-Dropper

The Magic of the Hopper Dropper – Jackson Hole Fly Company

Double your chances with a dry dropper rig: dry fly on the surface

Early Roots of Fly Fishing in South Devon

Three Early Season Dry-Dropper Rigs for Trout - The Fly Crate

Double your chances with a dry dropper rig: dry fly on the surface

When a dry-dropper rig should be your go-to nymphing tactic

Nymphing: How to read a fly fishing indicator - What you might be