Ostroy and Starr tell a story of "a pair of Robinson Crusoes":
Two elderly, largely self-sufficient gentlemen live on an island. Having only the most anemic impulses to truck and barter, their sole contact is the irregular exchange of dinners. Since both agree that meal preparation is onerous, they take turns. However, because dinners are exchanged so infrequently and because their memories are not what they used to be, these Robinson Crusoes cannot always agree on who gave the last dinner. On several occasions both have claimed to have provided the last meal. Each gentleman recognizes that this is a self-serving claim since this is what each would like to remember, but neither is sufficiently confident of his recollection to be sure of the truth. These disagreements have produced so much tension and ill-will that dinners are now exchanged even less frequently.
To attenuate this problem, the one who is coming to dinner next picks up a stone and paints it an artificially colored green to distinguish it from other stones and brings it to his host. At the next planning session for a dinner, the most recent host will be reminded by the presence of the green stone that it is his turn to be invited, and he will be expected to bring the stone with him when he arrives. Indeed, without receiving the stone the host may feel justified in turning away his guest as not having the required evidence of an invitation.
This quite rudimentary story reveals an essential feature of monetary exchange. Money is a commonly acknowledged record-keeping device. Here the only information about the past which has to be recorded is who gave the last dinner. Each gentleman "pays" for his dinner by transferring the record of this fact to the other.
That's a nice story which reveals something fundamental about money. The story is so good that I want to write a sequel to it.
Let us assume that in addition to these two gentlemen there is a fully self-sufficient accountant (an oxymoron?), called Monday, on the island. She has a lot of spare time and wants to help the gentlemen. Being elderly, the gentlemen seem to misplace the green stone once in a while, and this has led to a time-consuming search operation on more than one occasion.
Monday suggests that instead of using the green stone as a "counter", the gentlemen could rely on bookkeeping as a record-keeping device. Monday would take care of the bookkeeping. All the gentlemen have to do is to remember to inform Monday about any trades of dinners that take place between them.
Monday finds out that Nick Sr, one of the gentlemen, holds the green stone. She takes this to mean that Abraham, another gentleman, owes a dinner to Nick Sr. Both gentlemen agree on this interpretation, although they are not sure about the total number of meals prepared by each of them over time (what is one dinner between gentlemen!).
Nick Sr gives the stone to Monday, who buries it deep in ground now that it has become redundant as a record-keeping device.
The gentlemen don't know much about accounting. This is how they imagine Monday's ledger looks like:
Abe: 0 stone(s)
Nick Sr: 1 stone
This is how the actual accounts in Monday's ledger look like (being a CPA, she couldn't help using double-entry!):
Abe: -1 dinner
Nick Sr: +1 dinner
End of extended story.
What can we learn from my extension to the original story of Ostroy and Starr?
First, that green stones don't reside on accounts.
Second, that there is no transfer of a green stone, or anything for that matter, from Nick Sr to Abe the next time the latter prepares a meal for the former.
Instead, there is a "transfer" of a meal from Abe to Nick Sr. Monday will record this meal trade in her ledger by debiting Nick Sr's account with 1 dinner and crediting Abe's account with 1 dinner. This will bring balances of both of the accounts to zero. How do the gentlemen then know who is to be invited for a dinner next time? They ask Monday, and by looking at the entries she has made she will find out that it was Abe who prepared the meal last time and is to be invited next time.
Is there a need for Abe and Nick Sr to understand how the accounting works? Does it matter if they continue to think that Monday is transferring stones between the two accounts? I don't think it matters, at least not when the economy is so simple. The only tradable good is a dinner which is assumed to not vary in terms of utility derived from it – if Abe is not as good at cooking as Nick Sr, I'm sure he makes up for it by being an otherwise entertaining host.
We cannot yet talk about a monetary exchange economy in this case. (We need another sequel for that, which in one way or another is forthcoming.) But as Ostroy and Starr write, one could say that the gentlemen "pay" for the dinner by transferring the green stone to the host. So at least in some ways the green stone is comparable to money, or currency more specifically.
With this post I wanted to draw attention to the fact that bookkeeping in a double-entry format might look quite different from record-keeping where "counters" like the green stone are used, even when the object of recording – dinners given and received – is the same.
 Ostroy, Joseph M. & Starr, Ross M. 1990. "The transactions role of money," Handbook of Monetary Economics, in: B. M. Friedman & F. H. Hahn (ed.), Handbook of Monetary Economics, edition 1, volume 1, chapter 1, pages 3-62 Elsevier. ("A pair of Robinson Crusoes", p. 8-9) – There is a working paper version freely available here.