Line nomenclature

[Silver Creek]

I must be out of touch since I haven't bought a new line in a while.
Today when line sizes and weights are mentioned there is a Roman numeral or type number included What is this about? Thanks.

Sinking fly lines are rated by number as to "Type" designated by a number. The higher the number, the faster they sink. The sink rate is labeled in inches per second.

However, you can approximate the sink rate in feet for every 10 seconds, which is an easier to remember and use than the inches per second sink rate. For example, a Type I will sink at about 1 ft every 10 seconds, a Type II at 2 ft per 10 seconds, a Type III at 3 ft per 10 seconds, and so on. When fishing these lines just do 10 second count downs to get to the level you want to fish.

For example, if you are fishing a Type II line and want to fish at 3 feet, you would count to 15 seconds; if you had a Type III line, you would count to 10 seconds before beginning to retrieve the fly.

Although the conversion from inches per second to ft per 10 seconds is an approximation, it is much easier to calculate approximate time delay for the depth you want to fish at.