Transition Zone (T1/T2)
The area where athletes switch gear between disciplines: T1 is swim to bike, T2 is bike to run, and both count toward your finish time.
The transition zone is the physical area at a triathlon, usually a fenced-off section near the start and finish, where athletes' bikes are racked and where they change gear between the three disciplines. Every athlete passes through it twice: once from swim to bike, once from bike to run. Those two passes have their own names, T1 and T2, and the clock does not stop for either one.
T1: swim to bike
T1 begins the moment you exit the water and ends when you cross the mount line and start pedaling. In between, you run to your bike (often barefoot, sometimes over pavement or gravel), strip off your wetsuit if you wore one, dry your feet, put on a helmet and cycling shoes, and get moving. Wetsuit removal alone can eat a surprising amount of time for someone who hasn't practiced it, since wet neoprene doesn't come off the way it goes on. Helmets have to be buckled before you touch the bike, not after, which is a rule officials actually enforce.
T2: bike to run
T2 starts when you rack your bike back in its spot and ends when you leave the transition area on foot. Here the main jobs are swapping bike shoes for running shoes, sometimes changing a race number's position from back to front, grabbing anything you want for the run (a hat, a gel, sunglasses), and getting moving again. T2 is usually faster than T1 since there's no wetsuit involved, but it comes with a different problem: your legs have just spent thirty minutes to several hours pedaling, and running on them immediately feels genuinely strange the first time it happens. That specific sensation is exactly what a brick workout is meant to prepare you for.
Why transition is called the fourth discipline
Swim, bike, and run times get most of the attention, but transition time is a real, measurable part of every finish time, and it varies enormously between athletes at similar fitness covering the same distance. Two racers with nearly identical swim, bike, and run splits can finish minutes apart purely because one of them has a clean, rehearsed transition routine and the other is hunting for goggles or fumbling with shoelaces. Unlike the three sports themselves, transition performance has almost nothing to do with aerobic fitness. It comes down to preparation: knowing exactly where your gear is racked, laying it out in the order you'll use it, and having practiced the actual motions, wetsuit strip, shoe change, mount and dismount, enough times that they don't require thinking under race-day adrenaline. A beginner can improve transition time dramatically in a single practice session, which is not true of swim, bike, or run fitness.
Most race results list a transition split alongside the three discipline times, so if you want to see how much time is sitting on the table, your own past results are a good place to look.