The purpose of a stop
A stop-loss converts an undefined-loss trade into a defined-risk one. Its job is not to be 'right' about the market; its job is to make your downside knowable.
Where to place it
Place the stop where your thesis is invalidated, not at a round number or an arbitrary percentage. For a breakout trade that's the reclaimed resistance; for a mean-reversion trade it's beyond the recent low.
The wick problem
Stops placed at obvious levels (just under a swing low) are targets for liquidity-driven wicks. Offset your stop by a small buffer, or use a close-based stop on the next timeframe up to avoid getting wicked out on noise.
Not financial advice
This lesson is educational material, not personalized advice. Examples and case studies are illustrative. Trading carries real risk of loss — never invest money you cannot afford to lose, and consult a licensed professional for guidance specific to your situation.