Is weapon reach measured from the edge of the mount's space or the rider's?

Question

So, if a medium or small creature, like a PC, controls 25 square feet, (a 5×5' area) and a large creature, like a horse controls 100 square feet, (a 10×10' area) and tactical combat takes place on a 5' grid, (no, I'm not budging on that, yes I know it's optional) and melee weapons either have a reach of 5' or 10'...

During mounted combat, does a rider's attack extend from the edge of his mount's controlled area, or from the edge of the area he personally controls on his mount?

Or rather, put another way,

During mounted combat, is a rider assumed to control the same space as his mount, regardless of size differences?

Answer 1

Reach is measured from the rider

RAW

Nothing in the rules says that mounting another creature changes your size. So, according to the most basic reading of the rules, your size simply does not change. There are no secret rules and this is both the simplest and most RAW answer.

So that would make the answer to your second question a definitive "no". While mounted your character maintains its original size and thus the space they control also stays the same regardless of mounted state or mount size.

To your first question, it would seem the answer would be that you weapon's reach, attack origin, and anything else related to your character's position would also not change.

However, this does not rule out the possibility of the character moving squares on the mount itself to make attacks (instead of staying centered on the mount).Jeremy Crawford also said that when using a grid and miniatures you can "fit" the rider in any square occupied by the mount,and that you can move around on your mount using your movement (here).

RAI agrees

Jeremy Crawford also agrees in a Tweet (now just unofficial guidance):

Being mounted doesn't change your character's size.

Answer 2

I am unable to find a specific answer to your question by RAW (maybe somebody else can help), but having a look at a relevant rule may help:

PHB 198:

If the mount provokes an opportunity attack while you're on it, the attacker can target you or the mount.

The way I read that is that if your mount's movement of space (its occupied area) provokes an opportunity attack, it provokes one on you. I would argue that since this system works against the player, it should work for the player the same way, that is, even though it makes no sense physically that you may attack with the full reach of a maul from the tip of the horse, ruling otherwise would put the mounted combatant at a mechanical disadvantage (I am disregarding speed for the purpose of this discussion).

TL;DR as there is no RAW answer I was able to find, you or your DM may rule it either way, although given the above and persuasive precedent from 4e I would argue it is fairer mechanically to rule as if the mounted combatant occupies the full space of their mount.

Answer 3

Space Controlled

The size is not about physical boundaries the creature occupies, but to area the creature effectively controls.

The Size Categories table shows how much space a creature of a particular size controls in combat.

Which means, you and the horse are somewhere inside that area. And have control over it.

Moreover, it also means that unlike our mini's the creatures which stand frozen in whatever pose, our characters are are moving within the squares they occupy. Dodging missed attacks (That's why Dex is used in figuring out AC), take a step forward (inside that space) to strike, turning in the square to reach the opponent (5e doesn't have facing, you simply occupy the space). Since this moving around doesn't leave the space, it isn't movement; but the rules are meant to simplify real life.

Logical Assumption From Above

It would be reasonable to assume that you attack from the creature you've mounted's space. Your range would be the adjacent reach from the controlled area (adjacent squares if you have a 5 ft. reach and are using the grid variant rule) of the mount. As the mount could turn in the square at zero movement/skill check cost to bring you closer to the target.

Official Ruling (at the time) | Designer's Intent

However, there is now an tweet from Jeremy Crawford, which at the time was considered an official WotC ruling and now is just an insight into the designer intent, which is simply:

Being mounted doesn't change your character's size.

This would mean that rider can only attack from the space the rider occupies, which is more realistic but depending on the minis used, might be slightly harder to track.

Answer 4

For a small or medium creature riding a large mount, it doesn't matter. Here's why.

You can attack a creature in any square into which your reach extends in any amount (based on several rulings from Jeremy Crawford regarding areas of effect). If you only count from the middle of the mount, a 5 ft reach extends half way into all squares immediately surrounding the mount, so any of those squares is valid for targeting. A 10 ft reach would extend 5 ft further, half way into each square beyond that, making any of them valid targets. This is effectively the same thing as counting reach from the edges of the mount.

This does assume that the rider is in the middle of the mount, because nothing else really makes sense to me. One does not ride on a horse's shoulder.

Now what happens if a medium or small creature is riding a huge mount? If we assume that the rider is in the middle, then the rider's reach is effectively reduced by 5 ft. This makes sense to me from a practical standpoint. If riding on top of an elephant, you need a long weapon to reach anything on the ground. However, this starts to present several problems with rules interactions, such as the mounted combatant feat's ability to force an attack on the mount to target the rider. What happens if the mount is in the attacker's reach, but not the rider?

The result is that for large mounts, there is no issue, count it however you want, and the result is the same. For huge mounts, there is no good answer, further rulings from WotC are needed, and DMs should go with their gut in the meantime.

Answer 5

There's a rule for this that I've heard cited many times, and I now use it myself. While not strictly RAW, it has the advantage of being relatively unambiguous, and using existing systems in keeping with 5e's principle of keeping similar things the same.

A creature's reach is measured as if they were walking in a straight line to stand in the square they are targeting.

Thus, a 10-foot reach means you can hit any square you could step into using 10 feet of movement. As it's the rider making the attack, we measure from the rider. To explain why I do this, look at the extreme example: a 6-foot-tall human is riding a 40-foot dragon. There really is no way that the human can reach out and stab a 4-foot-tall halfling standing at the very corner of the dragon's space unless the human uses some movement to slide down the dragon's flank (rolling Acrobatics to look awesome doing it, no doubt) and make the attack from a more sensible distance.

Answer 6

According to the Player's Handbook, when a creature can opportunity attack your mount, they can choose to attack you instead, so you are treated as occupying the same squares that your mount occupies. If it applies that way to you being attacked, I do not see any reason for it to not work in reverse when you are attacking.