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As far as I can tell tremor shells and psychic shackles stack as they are different abilities.

Unless their rules state that they are not cumulative with other abilities that reduce movement or only certain keywords then yes, they do stack.

To be certain I'd want to see the text of the relevant abilities.

However, in general terms of you had two different abilities that said to halve movement then they would, provided there was no wording precluding it, stack. This would result in the movement value being quartered.

AFAIK you apply the normal rules (i.e. you halve, round [edit: up], then halve again [edit: then round up]).

I had the same question and e-mailed GW already. I know how my group would play it, but it is GW... You could send another e-mail and hope it goes into an FAQ.

Cleric wrote:
I had the same question and e-mailed GW already. I know how my group would play it, but it is GW... You could send another e-mail and hope it goes into an FAQ.

I don't see how it needs an FAQ... We have all the interpretive tools needed to resolve it.

Are there two specific rules that you have in mind that possibly throw up more complications?

They both say "halve its Movement characteristic" right?

This does not change the Datasheet though.

An argument can be made that you only can halve the movement characteristic once, as the datasheet does not change.

But YMMV.

Discuss it with your opponent or TO first.

If it says “movement characterist”, then there is only a single characteristic; the one written on the data sheet. So that’s what gets halved. You can halve it forty times, but seeing how the characteristic is what is being changed, it will always end up being X/2. Not X/2/2/2...etc.

 timetowaste85 wrote:
If it says “movement characterist”, then there is only a single characteristic; the one written on the data sheet. So that’s what gets halved. You can halve it forty times, but seeing how the characteristic is what is being changed, it will always end up being X/2. Not X/2/2/2...etc.

How do you come to that conclusion? Is there a citation?

If you can't change it then surely halving it the first time does nothing either, which would make no sense.

So, this kind of thing goes into the realm of mathematical logic. Let’s see if I can do this without fracturing my brain on a phone instead of laptop...

Movement characteristic is a set value. Say M (since that’s what GW used). You’re allowed to halve ‘M’. But if you have a second ability that halves ‘M’, you’re still only halving ‘M’. Not halving 1/2M. So:
P->Q (logic terminology) is representing M/2 (move characteristic halved) from the first ability
R->Q is M/2 from second ability
You don’t get to do P+R->Z. You get to do P->Q and R->Q so P and R->Q. What does this actually mean though? It means at the end of the day, the hard, written characteristic is being halved. You have two abilities that halve it. If you halve the half, you are no longer dealing with the characteristic. You’re modifying the already modified characteristic, which is an entirely new thing (and not actually the characteristic). This is the long, convoluted “wtf” reasoning that gets taught in the logic portion of trig.

Or...use another characteristic as an example. If a model as a Wound characteristic of 8, but takes 6 wounds, you don’t get to snipe it with an ability that says “automatically destroy a model with a wound characteristic of 2 or less”. The characteristic is still 8, regardless of 6 wounds being put on it. When you halve it’s movement characteristic, the characteristic is still M. So let’s use a move value of 6, as numbers are easier than letters for math. The attack halved the characteristic of 6 and you get 3. But when the second attack hits, it’s once again attacking the characteristic; which is what is written on the data sheet, which is 6. So you halve it from 6 to 3 and a second ability halves it from 6 to 3. Not 3 to 1.5. The only thing that would actually do this is if you have something that says “permanently change the move characteristic by “X”.

So that’s the logical, mathematics rationale that should make everyone go “what the hell did I just read?”

I sincerely doubt GW expects everyone to have done well at trig logic though, so they probably meant you can halve the half. But technically, logically, mathematically, you can’t.

I actually happen to be a maths graduate. I think you're working from a flawed premise.

If you halve your Movement Characteristic then your movement Characteristic is M/2 until the effect ends. We know that the characteristic is now M/2 and not M, otherwise you would move M.

If you halve it again, it becomes M/4.

I'm in the net result being (around) 1/4 of the original movement value.

As per pg. 175 'All modifiers are cumulative,..'

The biggest question is whether you halve the movement, halve it again and then round up any fraction, or round up after each halving. Personally I think you do all the division and then round.

Cornishman wrote:
I'm in the net result being (around) 1/4 of the original movement value.

As per pg. 175 'All modifiers are cumulative,..'

The biggest question is whether you halve the movement, halve it again and then round up any fraction, or round up after each halving. Personally I think you do all the division and then round.

I would tend to go the other way - you resolve each effect before applying the next one, which would mean doing the rounding if necessary after the first before applying the second. They're presumably being applied at the same time, which would mean that they would be sequenced to apply the effects.

 Stux wrote:
I actually happen to be a maths graduate. I think you're working from a flawed premise.

If you halve your Movement Characteristic then your movement Characteristic is M/2 until the effect ends. We know that the characteristic is now M/2 and not M, otherwise you would move M.

If you halve it again, it becomes M/4.

I’m trying to remember back half my life (17 years ago!). But I’m pretty sure that’s the way it works with mathematical logic. Logically, from the average human mind, is a different matter; you’d end up with the 1/4, as most people think. But while I don’t have a college mathematics degree, I scored in the highest graduate requirement exam percentile (99% on the GRE) for math. So I have the skill set, just not the need to use it and it’s rusty. But I’m positive that’s how it works. Again, math logic and human logic are two different things. And in this case, they disagree.

 timetowaste85 wrote:

 Stux wrote:
I actually happen to be a maths graduate. I think you're working from a flawed premise.

If you halve your Movement Characteristic then your movement Characteristic is M/2 until the effect ends. We know that the characteristic is now M/2 and not M, otherwise you would move M.

If you halve it again, it becomes M/4.

I’m trying to remember back half my life (17 years ago!). But I’m pretty sure that’s the way it works with mathematical logic. Logically, from the average human mind, is a different matter; you’d end up with the 1/4, as most people think. But while I don’t have a college mathematics degree, I scored in the highest graduate requirement exam percentile (99% on the GRE) for math. So I have the skill set, just not the need to use it and it’s rusty. But I’m positive that’s how it works. Again, math logic and human logic are two different things. And in this case, they disagree.

Game rule logic trumps both math logic and human logic. And, as Cornishman pointed out "As per pg. 175 'All modifiers are cumulative,..' "

 timetowaste85 wrote:

 Stux wrote:
I actually happen to be a maths graduate. I think you're working from a flawed premise.

If you halve your Movement Characteristic then your movement Characteristic is M/2 until the effect ends. We know that the characteristic is now M/2 and not M, otherwise you would move M.

If you halve it again, it becomes M/4.

I’m trying to remember back half my life (17 years ago!). But I’m pretty sure that’s the way it works with mathematical logic. Logically, from the average human mind, is a different matter; you’d end up with the 1/4, as most people think. But while I don’t have a college mathematics degree, I scored in the highest graduate requirement exam percentile (99% on the GRE) for math. So I have the skill set, just not the need to use it and it’s rusty. But I’m positive that’s how it works. Again, math logic and human logic are two different things. And in this case, they disagree.

'Math logic' Vs other logic doesn't come into this. It's all down to how you define the specific operation. And the in the context of the game, the most reasonable definition for that operation would result in M/4.

 doctortom wrote:

Cornishman wrote:
I'm in the net result being (around) 1/4 of the original movement value.

As per pg. 175 'All modifiers are cumulative,..'

The biggest question is whether you halve the movement, halve it again and then round up any fraction, or round up after each halving. Personally I think you do all the division and then round.

I would tend to go the other way - you resolve each effect before applying the next one, which would mean doing the rounding if necessary after the first before applying the second. They're presumably being applied at the same time, which would mean that they would be sequenced to apply the effects.

The second half of the sentance is '...you should apply any multiplication or division to the characteristic (rounding fractions up) before applying any addition or subtraction.

To me this would imply applying all multiplation and all division before the rounding.

Cornishman wrote:

 doctortom wrote:

Cornishman wrote:
I'm in the net result being (around) 1/4 of the original movement value.

As per pg. 175 'All modifiers are cumulative,..'

The biggest question is whether you halve the movement, halve it again and then round up any fraction, or round up after each halving. Personally I think you do all the division and then round.

I would tend to go the other way - you resolve each effect before applying the next one, which would mean doing the rounding if necessary after the first before applying the second. They're presumably being applied at the same time, which would mean that they would be sequenced to apply the effects.

The second half of the sentance is '...you should apply any multiplication or division to the characteristic (rounding fractions up) before applying any addition or subtraction.

To me this would imply applying all multiplation and all division before the rounding.

It could actually be read either way. Logic would normally say do the division and round the fraction afterward, but sequencing would tend to support going with rounding each division. I wouldn't be surprised though if Generally Accepted Practice is to do all the division before rounding.

The psychic power Psychic Shackles from the Indomitus Discipline specifically states that:

Ultramarines Codex Supplement wrote:A unit cannot be affected by both this psychic power and the Tenebrous Curse psychic power at the same time.

Tenebrous Curse and Psychic Shackles both halve a unit's movement characteristic. That they specifically called out that these two abilities cannot be used on the same target, I imagine that the implication is that they would stack. And, thus, Tremor Shells should stack with either Tenebrous Curse or Psychic Shackles (but, clearly, not both).

This is nothing to do with mathematical logic, it is to do with whether the unit's 'movement characteristic' is the value written under 'M' on the datasheet or the distance that the unit is currently able to move (including modifiers). Since models can move a distance equal to their movement characteristic I would say the latter, therefore you half it twice.

 Bilge Rat wrote:
This is nothing to do with mathematical logic, it is to do with whether the unit's 'movement characteristic' is the value written under 'M' on the datasheet or the distance that the unit is currently able to move (including modifiers). Since models can move a distance equal to their movement characteristic I would say the latter, therefore you half it twice.

The unit's 'movement characteristic' is as follows as per P.12 in the battle primer:

"Each unit has a datasheet that lists the characteristics, wargear and abilities of the models in that unit..."(1st graph).

"Profiles These contain the following characteristics that tell you..." (Bullet point 4 1st graph).

"Move (M): This is the speed at which a model moves across the battlefield." (Bullet point 4 2nd graph).

Yeah, the movement characteristic IS the distance they can move. If it didn't modify the characteristic then it wouldn't do anything the first time.