I said I would write about fly lines. I think it is Simon Gawesworth from RIO fly lines coloured string - and if you ever get the chance to listen to him, please do it. He is passionate about fly lines and helps explain them simply. 
Realistically though fly lines are a hugely complex, so I will just give my very brief overview on floating trout lines.   

Most fly lines sold are floating weight forward lines. There are a few double taper, spey, switch and sinking lines sold, but mostly they are floating WF. Lines are generally 90 feet, with a couple of lines used for tournament casters at 120 feet. 
All the top companies make very good lines 
Are they all the same? Certainly not. In our shop we often hear ‘I’d like a new fly line - a weight forward please.’ The angler might have had their old line for 20 years and have no idea what it is, nor anything about it.

We ask a few questions to try and find the right line for every angler.
The one tough question is: Can you cast?

Many are poor casters and will explain away their ineptitude with ‘You don’t need to be a good caster to catch a fish’. Whilst this is true, there is no disadvantage on being able to cast well.

These are the questions we ask to try and find the best line for each fly fisher's needs.
1.    What brand and model rod do you use? This question is to establish if it is a fast or slow rod and what sort of line it will need to load it.
2.    What weight rod is it? Some people don’t know….
3.    What line do you use now?
4.    How long have you had that line? 
5.    How do you fish? Short or long? Out of a boat? Into the wind?
6.    River or lake? Under the trees? Do you use a roll or bow and arrow cast?  
7.    Do you use a team of weighted/unweighted wet flies, delicate dry flies, nymph under dry?
8.    Do you look after your lines? Some people polish them every time they use them – others, like me, rarely clean a line.
9.    Do you have a line or budget in mind? 
10.    How good a caster are you? Oops, the delicate question again.
Each of these questions – and there are many more, helps establish the criteria to find the best line for each individual. Delicate dries and presentation lines are far different to hurling a large wet, quickly at short distance. And, as mentioned earlier we are only talking about floating lines at the moment.
Overweighting a rod – putting a #6 line on a #5 rod can be useful in at least two situations. For a novice it loads the rod faster, and secondly it is better if you are fishing short.
Fly line head length is far more interesting than one would think, and varies enormously. Given line weight designation is measured at 30 feet, it is interesting to note lines such as a RIO Gold has a head length of 47 feet, a RIO Grand has a shorter head of 38 feet, but is a full line size heavier, so in a #5 weight a RIO Grand comes in at 160 grains, not 140 as the RIO Gold is. The RIO Perception has a 36 foot head and at 30 feet it is true to weight, so should feel the same as the Gold. However, the shape of the head also varies with the Perception having a longer front taper.
In Scientific Anglers lines there are also similar variations. Trout taper (almost identical to SA's Expert Distance) has around a 60 foot head and true line to weight, MPX taper is 36 foot and is over weighted by half, so the #5 weight is 150 grains at 30 feet instead of 140 grains. The new Infinity taper has a 50 foot head and is also half a line heavier. The VPT has a medium length head at 44 feet, but a very long front taper.   
        
Let's look at a few popular lines from just two brands, models and what I think are their best applications.
1.    Scientific Anglers Trout taper. A very long head and taper of 69 feet. True to line weight at 30 feet. This is an outstanding line that delivers dries delicately and will roll cast and mend like a dream. You can lift a lot of line from the water and hold a lot of line in the air. It is almost identical to SA’s Expert Distance line, which casting students and teachers use. The Amplitude model is super slippery and has texture, which helps with distance and never needs to be cleaned.
2.    Scientific Anglers MPXS. This is a bit like the RIO Grand in some ways. Head length is 35.5 feet and is half a line heavier at 30 feet with weight concentrated at the very front of the line which helps load fast rods and turn over bigger flies more easily. The slippery, textured Amplitude model is the best of these lines and shoots like a rocket. 
3.    Scientific Anglers Infinity: This is relatively new line that has a long 50 foot head and is a half line weight heavier at 30 feet. Whilst is has most of the weight towards the tip, the front taper does allow for good presentations. The bonus in the long head is that it is excellent for mending on the water and you can pick up a lot of line when needed. This is difficult with short heads. The long head also allows you to hold a lot of line in the air for long casts and the Amplitude texture along with AST Plus has this line shooting into the next postcode. Rolls casts are a breeze and a camo tip is a bonus if you are worried about spooking fish.

4.    RIO Gold: This line has been around for many years. It has a reasonably long head length (47’) and a short front taper. It is true to line weight at 30 feet. This is a good all-round line and good value. It can deliver small dries, nymph under dry and small wets reasonably well. The long head helps roll casting and mending as well. It has welded loops on both ends and is two colours; moss, with gold running line. There is also a longer version used by tournament casters with a longer head and total length of 120 feet.
5.    RIO Grand: This has also been around for many years. It has a short aggressive head of 38 feet and is a full line size heavier at 30 feet. A better choice for modern fast action rods where loading the rod quickly is important. The weight is directed to the front and this helps turn over large and weighted flies much better than the RIO Gold. Tan running line with Camo head and welded loops both ends.
6.    RIO Perception: True line to weight at 30 feet and has a 36 foot head. A longer front taper than the Gold and much less aggressive than the Grand. A good all-round dry fly line with loops and three colours; Gray, Tan and Camo.

I could go on for many paragraphs about lines, but perhaps the best suggestion I can make is go and try a line if you can. In our store we have many spooled up lines on reels you can try with your rod - or ours. 

Airflo have just released some new lines as well, so ask us about them. 

Call us if you want a line, disagree with our thoughts - or have a query. 

Mike, Allan and Jules.