90m (100yds) spools.
The RIO® Dacron Backing (100yds - 30lb) is super high quality, just what you would expect from RIO. RIO's traditional Dacron braided backing features high strength and low stretch and is unsurpassed in quality.
A good compromise between thinness and the ability to make blind splice loops as suggested in some fly line set-up guides. It's easier to knot than brands that are very thin but have too tight a weave for a blind splice loop.