What is it for?
It exists so that a broken transmission does not become a card balance, which does not become a minimum payment, which does not become a decision made from fear two years later.
One event is survivable. It is the chain reaction that does the damage.
How much?
Start with what one month of your life actually costs, all in, including the bills that arrive annually.
One month of that is the first target. It sounds small and it removes most of the panic, because most emergencies are smaller than men imagine.
Three months is the second target. Six is the one people quote, and it is a good place to land eventually, but a man who aims straight at six often gives up at zero.
Where should it sit?
Somewhere boring, separate from the account you spend from, reachable within a day or two.
It is not supposed to grow. Its return is measured in decisions you did not have to make badly. A fund that is invested for a better return is not a fund, it is an investment that will be down exactly when you need it.
When should it be used?
For the unexpected and the necessary. Not for the predictable, which belongs in the budget, and not for the wanted, which belongs in savings.
Tyres are predictable. A hospital visit is not. Confusing the two is how a fund quietly becomes a holiday account.