I am going to replace the Shimano Acera X derailleur on my older Trek 920 but I am confused as to which model I should chose. The gear train is Shimano 3 X 9. There are two types of derailleurs listed. One is a "downswing" and the other is a "top swing". Since I am not familiar with their terms I am at a loss. I am trying to upgrade this bike that I have had since the 90's. Any help would be greatly appreciated. Thanks.

do you mean top pull vs bottom pull? Which is related to cable routing, does the cable to the FD come up from the bottom or down from the top tube?

This one will work either way, and comes with adapters for the three common seat post diameters: https://www.amazon.com/Shimano-FD-M3.../dp/B001ORZF94

https://www.amazon.com/Shimano-FD-M5.../dp/B002UZKUAI this one is a bit nicers, but you'll need to get seperate shims if your seat post is smaller than 34.9mm.

Top swing = low clamp. means the derailer swing above the seat tube clamp.
Bottom swing = high clamp. means the derailer cage swings below the seat tube clamp.

Top/bottom pull are more important differences.

Thanks for the answer. To answer your question is my problem. My derailleur pulls from the top. What I don't quite understand is their description of their products. If they are using the term "downswing" in the place of "bottom pull" then I would have understood. But not having their product in my hand to look at, I just don't understand. It looks like a language problem. Thanks again.

I haven't seen the term "swing" used with derailleurs, but there is a common distinction between bottom pull and top pull.

The bottom pull derailleurs route the shift cable along the downtube of the bike, and either above or below the bottom bracket, then up to the derailleur.

Top pull derailleurs will route the shift cable along the top tube, then down the seat tube, and pull the derailleur from the top side.

A third type of bottom pull derailleurs is primarily seen in vintage bikes and requires a cable stop to be integrated with the derailleur, and may require either vintage parts, or a clamp-on cable stop to be added/changed.

Links for newbies is a bit of a problem, but do you have an exact model of the derailleurs that you're looking at so we can see what you're comparing?

"Swing" and "pull" describe two unrelated things.

Just as dabac described, "swing" describes where a derailleur's cage is positioned relative to the seat tube clamp. The cage is above the clamp on a "top swing" derailleur and below it on a "bottom swing". Either will work on most bikes, except in cases where there's something preventing a derailleur clamp from being mounted in a particular spot. For example, I've got one bike whose water bottle bosses are positioned low on the seat tube, which can get in the way of the clamp on a bottom-swing derailleur.

"Pull" describes what direction the shift cable runs to the derailleur. If the cable routes from the bottom bracket (crankset) up to the derailleur and pulls it down, the derailleur is a "bottom pull". If the cable routes along the top tube, then runs down the seat tube to pull the derailleur up, it's a "top pull". If a derailleur can be cabled either way, it's a "dual pull".

According to BikePedia, a 1998 Trek 920 had a Shimano AceraX top swing, top pull derailleur:
1998 Trek 920 - BikePedia

A Shimano Altus FD-M370 would be a suitable 9-speed replacement:
https://www.niagaracycle.com/categor...ont-derailleur

I really appreciate the information. Thank you all for the help.
Tom

Get whichever one the bike had on it before. There is a reason why the manufacturer used the one that they chose. Every single time that I've ever tried to change I ran into a flared seat tube or water bottle cage mount or something.