In order to counteract the natural tendency of flylines to sink belly first (simple physics) some flylines are Density Compensated.

Bruce Richards of 3M writes:

"Our machinery can blend coatings together in whatever mix we chose. For our Uniform Sink lines (the first dens. comp. lines) we make the tip of the line with 100% of the denser of two coatings. As we progress up the front taper we blend in some of the less dense coating. This continues until the line is 100% of the less dense coating somewhere back in the belly. The result is a line that sinks fastest at the tip, less fast as you go back. We have tested other manufacturers dens. comp. lines and have found most have tips that sink slower than the bellies. For this type of line to fish properly, the tip has to sink FASTER than the belly or the leader and fly will hold the tip up and you will still have a sag in the line and low sensitivity."

Bruce adds by way of further explanation of how this works:

Envision two wires spliced together end to end, one of aluminum, one of tungsten. The aluminum wire is .050" diameter, not very dense. The tungsten wire is .010" diameter, very dense. A foot of the tungsten wire will sink faster than a foot of the aluminum wire, but weigh less. That is the same philosophy that should be used in DC lines.