I have always heard that Marth’s Neutral-B attack is effective for edge-guarding due to its low reach. It is often stated that its reach is lower than Marth’s other useful edge-guard attack, the Forward-Smash. This question gets asked quite often.
I decided to investigate the details of which attack actually hits lower.
I began by trying to determine which hits lower by inspection of the on-screen pixels. Sooner or later I found that using pixels is generally a bad idea because the camera angle leads to skewed results. Precise location values serve far better, as I only have to let the game do the work, and simply read off the values.
Images are still helpful in showing the general goings on:
The most downward hitbox of Marth’s Forward Smash reaches to 11592430 (y-units). Where as the most downward hitbox of Marth’s Neutral-B reaches to 10439884.
Conclusion: Forward-Smash reaches lower than Neutral-B.
Hitboxes sometimes behave differently depending on the terrain and other positional details. In this case, I have measured Marth’s downward reach when standing on the extreme edge of a flat surface (roll to the edge and turn around).