Instruction: Answer the given question by extracting information from the supporting document.

Input:
Question: What's the height of the backbone? And will it still look good using 2 of the 3 shelves even though there is a separate 2 shelf model?
Document: They are very small, and a couple were pressed together.2. It's important to use a level to mount the backbone of the shelf assembly. Very important to the final result.3. When screwing the brackets to the backbone, use a level across the arms.
I wanted somewhere to store our tv electronics, but not some bulky entertainment center. This is perfect. However, we decided to use only 2 shelves instead of 3 because first, turns out we only needed 2 shelves but also because our tv comes down quite a bit on the rack and 3 shelves would've looked kind of cramped. Clean and simplistic look and leaves you a lot more space than an entertainment center.
Easy to install. Works exactly as intended. Cable management is useless if you have way too much wires. Prob can only fit 3 Cat5 cables TOTAL. I have ps3, xbox, and their accessories on this and it is nice. I had to neatly cable tie the cables so they will still have a clean look while being in plain view. Shelves are not very big, but gets the job done.
Easy to install and great for holding DVD player or a PS3. I was able to install it myself, even though I am not a carpenter. Drilled a few holes and screwed it in. My stereo is a bit too large for the shelf, but was able to adapt it to hold the stereo onto the shelf.
I like the product. Easy to install though I recommend them using regular lag bolts and and Phillips screws for the shelves.
2 shelves for 30 bucks. 3 shelves for $60. Who came up with that? Maybe there's a reason, but it seems to me that I'm better off getting 2 X 2 shelves.
DVDs don't generate heat. The Panasonic's DVD feet just fit the glass, so there is a one-inch overhang on the sides. Still looks very nice, and they are at a height where I can stand to insert media.This is a very sturdy shelf system, and the hidden wiring is a great plus.
The product is good. Installation was easy and it is much sturdier then expected. I have a devices on one of the shelves, which adds a little weight (still less then 10pounds) and it holds perfectly.
Easy to install but if you are looking to hide several cords this unit will not do that. The cord area is a very small area along both sides of the base. The shelves look nice - but my cords are still all over and unattractive.
Had to return this as the shelves were just a  bit too small and the weight of my components was over what the max weight of this shelving will hold. Max is 17 1/2 lbs. Too bad because I really liked the look of this in person, so if you are considering it and the weight of your components is under 17 1/2 lbs, I would recommend this shelving system. Great price and look if you just have one or two components to put on the shelves.
Answer: Back bone is 22 3/4  long . I guess it depends what and how big it is that your putting on the shelves. It's good to use two and have a third option. Just take pice of cardboard and make a templet. To see how it will look