AC Power cord with duck head question, difference?

The third prong on the cord is for grounding, which you don't get with the two-prong duckhead.

No, that is incorrect. Mostly because surge protectors in general are completely worthless at protecting anything valuable enough to be protected. The thousands of joules they say they protect against are generally pretty small when it comes to the kind of power you get in your average (significant) power surge. A good lightning strike induced surge can probably easily be hundreds of thousands, if not millions, of joules depending on how far you are from the source. So how some dinky little 5,000 joule protector is supposed to do any good is a real mystery to me. And that's ignoring the fact that every little surge a surge protector stops, tends to chip away at its overall effectiveness, so each time it takes a hit, the size of the next hit it can take goes down.

I'll let an electrician or electrical engineer explain the finer details of the benefits to a grounded connection. I'll just leave it at: If given a choice between grounded and ungrounded plugs, grounded is preferable.

Any good surge protector has a three-prong grounding plug of its own, and anyone with any sense will plug it into a properly-grounded three-prong outlet. Whether the ground actually gets extended to your MBP depends on whether you use the three-prong cord or the two-prong duckhead. Use the cord and your machine is grounded; use the duckhead and it isn't. As Scott says, grounded is better. The same is true if no surge protector is involved, and you're connecting directly to a wall outlet — assuming, of course, that the outlet itself is properly grounded. To verify that and to detect wiring problems in your home or office, you can use an inexpensive outlet tester like this, available at any hardware store:

http://www.amazon.com/gp/product/B0002O1E7W/ref=pdlpo_k2_dp_sr_3?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B0 000WU4Q8&pf_rd_m=ATVPDKIKX0DER&pf_rdr=0GNJH0J19J2WSN46S1AB

Just for clarity, the little corner piece with the two prongs is usually called a duckhead. The cord with three prongs is not called a duckhead. Don't ask me why, in either case — that's just how it is.

And yes, I'm saying that whenever a three-prong outlet is available, it's best to use the three-prong cord instead of the duckhead. If only a two-prong outlet is available (as in old buildings with old wiring), then you might as well use the duckhead, because there's probably no reliable ground to take advantage of even if you get a three-prong-to-two-prong plug adapter.

powerbook1701 wrote:
so what you are saying is that it is better to use the extra "duckhead" cord from the white power square thing regardless..

Based on experience, I use the 3-prong cord all the time. The reason is that I used to leave the 2-prong hinged duckhead attached, and what happened is that the prong hinge inside wore out and started arcing and failing, and eventually died in hazy smoke and brown scorched plastic. Another duckhead I had for an old iPod USB adapter was so intermittent you had to have it just so or the charge light would not come on, and it was out of warranty so Apple wouldn't replace it. I've concluded that the hinged duckhead is far less reliable simply because it has a moving part. And the way plugs get used, you're always straining that poor little prong hinge that your entire power supply is dependent on, by hanging that big power brick from it, and of course it gets tugged and torqued until one day...

The extension cord never gives me trouble; since I usually don't need the whole cord I tie up a portion of the length. I keep the 2-prong in my bag for the rare times I need to plug into an old ungrounded outlet.

I forgot to mention that if the duckhead dies on you, Apple will not sell you that part. You have to buy the whole dang adapter again.