The effective top tube (ETT) is the horizontal distance from the seat tube to the head tube. It measures how long your cockpit is seated and pedaling — unlike reach, which measures it standing. It's "effective" because it's measured horizontally even though the actual top tube may be sloped. It depends on the seat angle: that's why two bikes with the same reach can have different ETTs. On the modern MTB it weighs less than reach, but it helps to understand the seated pedaling position.
It was the king number for picking a size for decades. Today it has handed the throne to reach, but understanding the difference keeps you from confusing two measurements that look the same and aren't.
Reach and ETT both measure "how long the bike is" — but one standing and the other seated. Confusing them is the cause of half the muddles when comparing geometry tables.
The ETT runs from the seat tube to the head tube, measured horizontally at the height of the top tube. It represents the distance between your saddle and the handlebar, that is, how stretched out or tucked in you are seated. Since almost all modern bikes have a sloped top tube, the "effective" (horizontal) value is given instead of the actual tube length, so that it's comparable.
Both measure the cockpit, but from different points. Reach starts at the bottom bracket and runs to the head tube: it's your standing position, and the saddle doesn't affect it. ETT starts at the seat tube: it's your seated position, and it does depend on the seat tube angle. A steeper seat angle shortens the ETT (brings you closer to the handlebar) without touching the reach. That's why they're not interchangeable.
On mountain MTB it matters less than reach: the position that decides control on a descent is the standing one, and saddle height is adjusted with a dropper post. But ETT is still useful when you pedal seated for a long time — XC, marathon, gravel, road — because there your comfort and efficiency depend on that saddle-to-handlebar distance. As a reference to understand a bike, it's worth it; as the sole sizing criterion in MTB, it fell behind reach.
The horizontal distance from the seat tube to the head tube. It measures your cockpit seated. "Effective" because it's given horizontally even though the tube is sloped.
Reach = cockpit standing (from the bottom bracket), doesn't depend on the saddle. ETT = cockpit seated (from the seat tube), depends on the seat angle.
Less than reach in MTB. Useful for the seated position (XC, gravel, road); not as the sole sizing criterion.
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