Just a bit further in explaining now?

Question

What is the whole purpose of going back 3/4" on this joist? See I am attempting to build a deck and I placed the joists every 24" [this was told to me by a cabinet maker] and once I finished the floor deck the deck was moving up and down and was not sold. My deck is 10X12 and the deck planks are 10' horizontal. So i came to the conclusion I did not place the joists
..............................................................................................................
This was written by locksmith:
You measure 15 1/4", from the outside of your rim joist. That is where your first 16" on centre is. Measure back 3/4", mark a line here with a square, and an X on centre.You may find it easier to drive a small nail [only in an inch, or so ] close to the line, so that when your tape is hooked on the nail the tape clip is exactly at the outside edge of the line. Now pull out your tape, keeping it square and tight, and make make V mark at all the stud [ 16" ] marks indicated by your ta

Answer 1

You learned a valuable lesson, don't ask a cabinet maker how to make a deck and don't ask a deck maker how to hang a cabinet. And yes 16 inches on center meaning from the center of one joist to the center of the next is correct.
The explanation from locksmith is saying that when you measure from the outside (should actually be the inside) of the rim joist 15 1/4" you will be at the center of the next joist (you could also just measure 16 inches from the center of the rim joist and be at the same spot) He has you move back 3/4" (half the width of the joist) to mark the spot where the edge of the joist will sit. This way when you go to put all of the joist in you know where the edge is and the center is and are less likely to place the joists in the wrong spot.
An easier way would be to just measure 16" from the outside of the first joist and mark off that spot, that will be the outside edge of the next joist (than mark at 32", 48" and so on). Just make sure that you remember that the line is for the outside of the joist.
Also make sure that the joists are hung using joist hangers and not just toe nailed into the edges, and if the deck is connected to a house you need to have a ledger board that is bolted through the wall (just a couple of other common mistakes).

Answer 2

12 ft. is a long span for a deck joist, especally on 24 in. centers. the easiest fix offhand would be to pull planks aff at 4 and 8 ft.get 12 in concrete pads and put down 4 ft. from each rim joist. cut 4x4 posts and screw to joists. best to pull up on joists while screwing because pads will settle. screw planks back down

Answer 3

He's also telling you that to make them 16" on center, mark 15 1/4, 31 1/4, 47 1/4, etc. so that when you line the edge of the boards on the marks, the centers will be on 16".

Answer 4

He is trying to tell you that the joists should be 16" on center. Whoever told you 24" is an idiot.