This brief article is the first in a series highlighting important points taken from the seminal book on fly lines written by Mr. Bruce Richards titled MODERN FLY LINES.
This is a must have book for the serious fly fisher.

Five types of cores
1) Braided Multifilament Nylon-made of braided filaments.Tensile strengths of twenty to forty pounds. Limp material suited for freshwater fishing.
2) Braided Monofilament Nylon-made of braided single strands of monofilament. Very limp but becomes stiff with added coating. More line memory and designed for saltwater use.
3) Single Strand Monofilament- with clear coating creates a clear line. Can be designed for fresh or saltwater.
4) Dacron- made of braided multifilament polyester.Used for off-shore big fish or tournament distance casting. Most commonly used as backing attached to fly line. Less stretch.
5) Kevlar-no stretch leading to memory problems. Poor coating adherence.

Doc Frangos, FFI MCI
Summary from Modern Fly Lines
by Bruce Richards