When you are building a performance exhaust system, two very important parts are the slips and transitions. Slips, both single and double help to reduce leaks and create a seal that is removable. Double slips are better than single slips in reducing leaks when you need a joint that can be easily disassembled. It is important to weigh out the difference of an easily disassembled exhaust, vs a leakproof sealed system. Transitions are engineered to get the best advantage of moving from one size tubing to the next.