Basic Points

• A left tail is a reference to how much money can be lost in an extreme situation for a position or portfolio. 
• A tail is generally considered three standard deviations (about 99.7%) or greater. But we can think of two standard deviation tails (about a 95% chance) as something that can still hurt us a great deal—absent a dynamic risk management plan.
• In the big picture, it is dramatically safer to be long a tail rather than be short a tail. 
• Being long a tail means that you only have a tiny amount of risk at stake but could make a fortune if things go your way (even though there is only a fraction of a percentage chance of that victory being the case). 
• Many traders fall for the trap of wanting to write these [full 3 standard deviation] tails naked because they see the accuracy associated with that. Quite plausibly, they could be right 300 times in a row without a single loss. They are not wrong. But if they do have a left tail event when they are exposed, they can suffer extreme loss.

All Levels: Risk Considerations

In this knowledge base, any option strategy which uses uncapped risk (otherwise known as undefined risk) is marked as advanced or expert. These strategies simply should not be attempted with real money until they are understood very well, and even then there are fundamental risks of the nature that only position size can really control. And then that control has limits, regardless of skill. 

Any position with undefined risk must be taken with maximum seriousness. This includes having measures in place to make sure that it is not truly undefined, such as stop-limits (with the underlying security) that would open a defensive position during extreme situations.