bike fitting: part two, effective top tube

Effective top tube, often listed as ETT. This is a great number to look at to pick a frame that fits, but it has some shortcomings.

ETT is the distance from the center of the head tube (the short tube at the front of the bike where the fork goes through to steer) along an imaginary, horizontal line to where that line intersects with the seat post. You can try to measure this for yourself, but chances are that the manufacturer of your current bike has this listed somewhere, so look it up

This measurement tells you how far the front of the bike will be from the saddle, ig. how "long" your bike is. If all other angles and measurements on a bike remain the same, you could pick a frame size entirely based on ETT. That is not the case, to the extent that bikes among models vary from one model to another and the approach that manufacturers' take from one year to the next evolve over time. It's still a good place to start if you know what kind of bike you're looking at.

For reference, a medium frame cross-country mountain bike frame might possess an ETT of 590 to 615 mm, maybe longer, depending on the manufacturers' philosophy. modern bikes are generally getting longer in favor of a longer front end to the bike, which is usually tempered by a steeper seat tube angle (which makes the seat more forward on the bike) and a shorter stem length (which brings the bike backward).

My point is, you need to take ETT, like everything else, in the context of the rest of the bike. Once you find an ETT that works for you, you can use that number, with some caveats, to find a frame that will fit you.