Considerations about key definitions

Round or just-below?

Prices like $97 and $490 can be considered both round, since they only two significant figures (non-zero digits.

Naming choice:

Unconscious rounding of just-below prices is reliable?

Most of time, we don’t need to mention which is the round price related to a just-below one. It is straightforward to realize that the round price related to $198 is $200 and the round price related to $37.99 is $38.
This happens because our brain knows unconsciously how to round up $198. The problem is that sometimes the brain forgets to perform the rounding. More details in building blocks and also in the ABC of rounding.

When there is more than one rounding possible:

The most important one is that “discounts” are shown often as percentage of the round value. For instance, $2 is 1% of $200, while $0.01 is only 0.005% of $200. Note that discounts as percentage are also tiny.
Although our brain knows how to round up, some just-below prices admit more than one rounding. For instance, $579 can be rounded up to both $580 and $600. If we round $579 up to $580, we have a “discount” of approximately 0.17%. If we round it up to $600, the “discount” will be 3.50%.
The “discount” as percentage is used whenever there is more than one rounding possibility. In the solution proposed by this movement, the valid roundings are those that result in “discounts” less than a specific limit. And the selected rounding is the greatest valid one. By default, the “discount” limit is 5%, but it can be lowered if desired.
Using a limit of 5%, both $590 and $600 are valid roundings (0.17% < 5%, 3.5% < 5%). The selected rounding is $600.
In case of $18.80, the $19-rounding is valid (“discount” of 1.05%), but the $20-rounding is invalid (discount of 6%).
Something, even if there only one rounding possible, this rounding can be invalid. Example: $370 (discount of 7.5%). For this reason, $370 is not considered a just-below price.
PS: But $370 is a round price, because it has only two significant digits.
Note that is $9.5 is a round price, (discount of 5%).

Complicated prices:

Saying that significant figures are the non-zero digits is a useful simplification when dealing with those prices that are round for sure. Actually, if there are zeros surrounded by by non-zero digits, these zeros are also considered significant figures. For instance: $1009 has four significant figures, $505.05 has five.
For the prices that are round for sure, we can say that the significant digits is equivalent to non-zero digits, because these prices has only one or two significant digits and, therefore, they don’t have space for zeros surrounded by non-zeros.
Prices with four or more decimal figures are named complicated. Besides $1009 and $505.05, $23.45 and $358,697 are also complicated.
A grey zone is the three-decimal-figure prices. $476 should be considered round or complicated? I would call it complicated. And how about $12.50? I would also call it complicated, although I’m less sure about it.

Round Price