This is my pressure treated 2x4 used for the framing of your picket fence. Now on this picket fence the frame, the bottom, and the top are pressure treated 2x4's. Now since this is a 24 foot long fence we will need 6 - 8 foot 2x4's. Now when you measure for your top frame piece make sure that you measure from the center of your 4x4 post to the next center of your 4x4 post. This will give your frame here a nice anchor and support. Now on the bottom portion of this fence, on the bottom frame, you measure from the inside of your 4x4 post to the inside of your next 4x4 post.