Bilateral breathing is a freestyle term made popular in swimming circles by Total Immersion, but the idea is much older.  Pure bilateral breathing is when you breathe every third stroke (one arm pull equals one stroke), causing you to breathe on alternating sides (left then right, then left again). 

Let’s look at the benefits, unfortunate reality, downside of bilateral breathing, and the bottom line.

First with the good: Breathing to both sides helps encourage roll to both sides.  Even roll helps position the arm relative to the core to get the most of pulls from both arms. 

Now the Unfortunate reality. Many swimmers who are bilateral breathing roll well to the side they are breathing on and not on the non breathing side.  This means you simply alternate which side they are rolling to and swimming relatively flat on the opposite side.  I’ll grant this is better than only rolling to one side, but it is not the end result we are looking for. 

You must work to ensure you are rolling to both sides at all times, and to be honest, you can do that with or without bilateral breathing.

On the downside of a pure bilateral (breathing every third stroke) is the tendency to build oxygen debt. Distance swimming at your best effort has high aerobic demands and breathing every third stroke can slow down your oxygen intake.  This is easily fixed, rather than breathing a pure bilateral, I recommend breathing two or three times on one side, then putting your head down for three and then breathing two or three times on the opposite side.  This maintains the benefits of bilateral breathing without the oxygen debt you might build with a pure one breath per side.

 The bottom line?  For me, I recommend bilateral breathing to all of my athletes using a breathing two or three strokes on each side then switching to the other side. I also recommend doing a full set every day breathing exclusively to the weak side to help even out the swimmer.  I personally, to this day, do all of my pull sets to my weaker side.