That is a good question and is always a mystery to many new fly fishers.
The leader has a butt section that is the large diameter line with the loop on it. This is the end that you fasten to the fly line. The other end that is much smaller in diameter is the tippet end. On the package the leader came in is listed the tippet size of the leader in a X size. So you could have a 9' long leader that tapers to a 5X tippet. The tippet diameter is also stated on the package. The small diameter tippet is from 20" to 30" or so long. As you tie on flies you use up the tippet until it is quite short. You then take tippet material that comes on spools and tie on another tippet to make it a 9' leader again. Each time you use up the tippet you tie on another one. This allows you to use one leader for a lot of fishing. Make sure you replace the tippet before it gets so short that your leader end begins to get bigger.
If you are using a 5X tippet and want to use tiny flies you may need to add a smaller tippet to the existing leader to change it from a 5X tippet to 7X. The larger the X number the smaller diameter the tippet is. You would take some 7X tippet material from the spool and tie it to the 5X leader and you now have a leader with a 7X tippet.
| Tippet Diameter||Tippet Size||Pound Test||Fly Size|
NOTE: Leader diameters and strengths vary from manufacture to manufacture and with different materials or processes.