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Depends on what you will be smoking as well as whether the racks are removable. For example, if they aren't removable but you'll only be doing ribs, you can make them 4 inches apart. If you'll be doing Boston Butts, you'll want them about 6 inches apart. If the racks are removable, then you can go with just about whatever spacing you want as you can move racks to whatever slots are necessary for what you are smoking.