The internal vent system looks like a straw inside the bottle, and it is made up of a round insert for the top of the bottle, plus the long, straw-like vent that extends the whole length of the bottle.
The standard neck bottles are available in 2 ounce, 4 ounce and 8 ounce sizes. The wide neck bottles are available in 4 ounce and 8 ounce sizes. All of the Dr. Brown's Natural Flow bottles have a clear silicone nipple. You can purchase various flow speeds in the nipples.
Dr. Brown's Natural Flow bottles cost $5 to $6 per bottle.
The classic shape of these bottles, both standard and wide neck, makes them easy to carry in most diaper bags, or in the insulated bottle bags that come with many diaper bags. The size also works in all of the bottle warmers I tried. However, I had a lot of trouble with milk leaking out of the top of the bottle when I used a bottle warmer. It worked better to take off the lid and the vent before warming the bottle.
These bottles rarely leak as long as they're put together properly. Assembling the pieces isn't hard, but in the middle of the night when you're tired, it's easy to forget a piece. You cannot use these bottles without the vent pieces. They leak profusely without the vents.
All of the pieces are dishwasher safe. I had no trouble keeping these bottles squeaky clean by using a regular dishwasher basket. The vent pieces tend to get yucky quickly if you forget a bottle in your diaper bag, though. Rinsing the pieces after use when possible will help avoid that problem. A small bottle brush is included if you need it.
While these bottles take a little extra effort to put together, and are more work when you need to open them up in public, they do work very well. If your baby needs a vented bottle to reduce gas or colic, the extra effort is worthwhile.