Two ten sided dice. One dice is a different color from the other and equals the "tens" spot. The other equals the "ones" place.
If we say that there is a percentage needed in order to determine if something IS going to happen, you start from the bottom up. So, say there is a 25 percent chance you might successfully throw a rock at a fleeing orc. roll the percentile dice and if the result is 1 to 25, you have hit that orc. Great work. If you roll anything 26 or more ("00" being 100), you missed.
It seems to me, that most things are written to favor the chance of it happening. But what of an option being expressed in the negative? For example, what if the option is giving that you have a chance to know how to read a scroll? There is a 25% chance you will know how. That option is expressed in the positive. You "will" know how to do it within 25 percent.
But let's say that even if you do have a successful knowing of the spell, a followup option says that there is also a 25 percent chance the spell will be messed up , mispronounced, etc... and fail.
In both cases you could make the case that if viewed in the positive ie. the result "will" happen in a roll of 25 or less. You would expect to start the roll at the bottom meaning, if the roll is 1 to 25, the result occurs. Any number rolled over the 25 is a negative or does "not" occur.
OR, I have seen it shown that having a 25% chance to know means there is a 74% chance to "not" know and the dice is then interpreted that any result of 1 to 75 indicates not knowing the spell.
Or in the case of spell failure, A 25% chance of spell failure or a 75% chance the spell will not fail and rolling a 1 to 75 indicates the spell succeeded. Anything rolled over 75 indicates spell failure.
As for me, I don't like the wavering view. It makes me seasick.
I have it that anything requiring a percentage roll starts from 1 to achieve it. Regardless of positive or negative result.
So, if the roll is a 50% chance to yank a stone gargoyle off from over a doorway, then the roll will be 1 to 50 indicates pulling it off (success) and anything 51 and over is a failure, it did not come off.
If there is a 35% chance that a PC gets bit on the butt by a flying fish while at the lakeside, then a roll of 1 to 35 indicates getting bit (success) and anything 36 and over is not bit (failure).
I think the reason some folks like to flip flop the odds is because they think they will get a better result if the result they want to have happen will happen on the low end of the roll.
Other folks obviously can and will do what they want with percentile dice, but at my table, that's how we roll. (heh heh)