This is by no means meant to be a compete tutorial on Pressure Treated Lumber. A complete presentation of all of the information is left better to manufacturer's websites or like this one from the Southern Pine Council. I just want to introduce you to the idea that not all pressure treated lumber is the same----most people think that "pressure-treated" is "pressure-treated." The reality is that not only are there many different "grades," of pressure treated materials, there are many different types of chemicals that can be used to treat the wood.
One thing that will be true of them all is that each piece of lumber will have a tag stapled to it that explains what it is. 
The tag will tell you how much preservative is in the material, what type of preservative was used----along with a bunch of other information----including the year it was treated and where.
It is important for the installer/builder to be familiar with what all this labeling means so that the correct materials get used for the right job.
The Grade of pressure treatment refers to the "amount" of preservative that is required to be forced (under pressure) into the piece of wood----per board foot. These percentages run from 2.5 ounces per board foot for pilings used in salt-water (marine) applications, to .25 ounces per board foot for some deck railing and surface installations. When you get up into the higher number of ounces per board foot there are only certain species of wood that will actually accept that much preservative----Southern Yellow Pine for example. (Don't tell that to the NW timber industry however.) For all its other great characteristics, Douglas Fir is lousy at being pressure treated. For most purposes, wood that is going to be in contact with the ground will have a saturation level above .40 ounces. This is very typical of most deck structural posts, beams and joists. For wood meant to be buried----as in Permanent Wood Foundation Systems (PWF's)----we want to see .60 ounces per board ft.
The problem is that because most people---especially "Weekend Warriors"----don't know the differences between all these grades of treated wood, we end up with the wrong grades being used in the wrong places. Another factor is that materials designed to be buried are only available at real lumber yards. You are not likely to find them at Weekend Warrior hunting grounds like Home Depot, Lowes, or OSH.
Here is a picture of your garden variety 
.40 Douglas Fir pressure treated 6x6. Because the treatment of this wood "species" (at this grade) can't penetrate all the way to the center of the wood----the center just rots away.
So when you are using pressure treated lumber---make sure you have the correct grade and warrior for the right job.
Charles Buell
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This is great information! It's definitely something I'll keep around for reference in the future!
DM
Charles, great explantion on the pressure treat wood. We all have different expertise here on AR.
Whew! Lots of info. However, nothing happens when I put my cursor over the smiley face.
Dan, thanks for stopping by.
Frank, thanks
Jean, thanks for stopping by and letting me know about the "smiley"----all better now:)
Charles,
I never knew there we different grades of Pressure Treated Lumber. Makes sense, but never thought about it before.
Sean Allen
I'm telling ya, we should be able to get some kind of CE credit for reading your blog, which I find to be one of the most informational out there. Thanks for sharing your knowledge! It definitely helps me know more - at least to know enough to ask the right questions.
I agree with Susan. We do need credit for this Buell course!
Sean---there are actually at least 6 grades that I am aware of.
Susan----you are too kind---just trying to give everything away:)
Barbara, how many would you like? Can I charge your credit card?:)
Again a wealth of information. I'm glad I happened onto your blog. I am always both informed and ammused by you (love the smiley faces) I hope since I put my ... I don't know what grade... posts to me deck on poured footers and water sealed them... they are not rotting from the inside and I will not "fall off" my house one day while standing on my deck... ouch.
*Note to self for future trips to Lowe's.
Thanks for the lesson. Will cutting the lumber have any affect on the integrity of the treatment? I am usually surprised if I see pressure treated lumber in use by the homeowner. Using the correct type would be asking way too much.
Tammy thanks for finding me---hope you continue to enjoy!
Steve, all cuts are supposed to be treated by hand in the field with the preservative appropriate to the grade---type of treatment.
Just found your blog, love it, it has great information and it is fun to read
I have to remember this one "Weekend Warrior hunting grounds"
Anne, glad you found me---enjoy.
Charlie,
I love it when people have old railroad ties and figure they will never go south. One of the biggest colonies of dampwood termites I have ever found were in such ties.
I am assuming you took this shot at my place and for the record: this was the look I was going for. Why are you not seeing the design element of the rotting from the inside out-- it is so obviously a metaphor for life. duh cheryl
Steve, the suseptibility of railroad ties has long been known, but people tend to think that pressure treated doesn't have the same problems; and, actually it doesn't it the higher grades. The problem is that most people think it is all the same.
Cheryl, yup---snapped the picture when you weren't looking----you were probably off selling houses or something:)
Hey Dad I have a friend that put up a deck last year with trex flooring and cedar raiing (pressure treated but I am not sure to what grade). Looked good initially but one year later and it looks like it caught a "mold" we sanded it and put another coat of protectant but I am thinking we'll be doing it all over again next spring. Any thoughts or suggestions? Is cedar a good wood to use for outdoor railing work?
Klee, cedar is OK---try to find a stain/presservative that has a fungicide in it----pretty difficult to keep it totally knocked down----but better than painting it in the long run.