Imagine a community where the money is based on the integers running from 1, 2, 3, …, up through 21,000,000. At any given time, one person owns the number 8, while somebody else owns the number 34,323, and so on.
In this setting, Bill wants to buy a car from Sally, and the price sticker on the car reads, Two numbers. Bill happens to be in possession of the numbers 18 and 112. So Bill gives the two numbers to Sally, and Sally gives Bill the car. The community recognizes two facts: The title to the car has been transferred from Sally to Bill, and Sally is now the owner of the numbers 18 and 112.
In this fictitious community, an industry of thousands of accountants maintains the record of ownership of the 21 million integers. Each accountant keeps an enormous ledger in an Excel file. The columns run across the top, from 1 to 21 million, while the rows record every transfer of a particular number. For example, when Bill bought the car from Sally, the accountants who were within earshot of the deal entered into their respective Excel files, Now in possession of Sally in the next available row, in the columns for 18 and 112. In these ledgers, if we looked one row above, we would see, Now in the possession of Bill for these two numbers, because Bill owned the numbers before he transferred them to Sally.
Besides documenting any transactions that happen to be within earshot, the accountants also periodically check their own ledgers against those of their neighbors.
If an accountant ever discovers that his neighbors have recorded transactions for other numbers (i.e., for deals for which the accountant in question was not within earshot), then the accountant fills in those missing row entries in the columns for those numbers. Therefore, at any given time, there are thousands of accountants, each of whom has a virtually complete history of all 21 million numbers.