Ladder – In a ladder serial number, every digit is one number higher (or lower) than the previous digit.
Really the only true ladder is a serial number than contains a letter then 12345678 followed by another letter. The true ladder in its purest form is only printed once for every 96 million notes in a standard print run.
True ladders are rare and command a premium. It is easy for someone to spot serial number 1 and realize it is valuable. It takes a much keener eye to consider saving a ladder.
There are also many other examples of ladders. Some people would consider A00012345A to be a ladder. Others might call D00034567A a ladder.
The next closest thing to a true ladder in terms of value is the true reverse, backwards, or step down ladder. It looks like A87654321A in its best form. Reverse ladders are fun, but they are always less desirable than up ladders.
A cheap form of a ladder comes in the “broken” ladder. It reads something like A01200304A. Some people like these, but they are really just more affordable versions of what people really want.
Some scam artists have recently tried to push the “scrambled ladder.” The scrambled ladder consists of 8 unique numbers, like A92378514A. These notes don’t carry a premium. A general rule of thumb is that if you need an encyclopedia and a calculator to determine why a serial number is important, then it probably isn’t important.