A stop-loss is an order placed (or a level mentally committed) at a price worse than your entry, that closes the trade if hit. It defines, in advance, the maximum loss you'll take on the position. Combined with position size, the stop-loss determines your risk per trade in absolute terms.
The stop has two jobs. The mechanical one is risk-capping: hit the level, exit, take the predetermined loss, move on. The psychological one is more important: it lets you enter a position knowing exactly how badly it can hurt, which means you can size correctly and not stare at the screen during the trade. A trade without a stop is a trade you can't size and can't walk away from.
Stop variants and their failure modes: - Hard stop in market — broker fires automatically. Best discipline, occasional slippage. - Mental stop — only as good as the trader. Most "mental stops" become "let me give it one more candle" — which is how 1R losses become 4R losses. - Stop too tight — gets stopped on noise, doesn't let the setup breathe. Wins look great, win rate looks awful. - Stop too wide — survives noise, but the winner has to deliver 3-4R just to break even on R:R. Now your win rate has to be insane. - Moving the stop further away mid-trade — the cardinal sin. It transforms a 1R loss into "we'll see, possibly 5R." Almost always tilt.
Stop-Loss is the discipline that, ten years in, distinguishes the trader who's still trading from the trader who quit. It's not about being right — it's about being wrong cheaply. Every wrong trade with a respected stop costs 1R. Every wrong trade without one costs whatever the trader couldn't psychologically tolerate, which is usually 3-5x the planned risk.
In TradeOnyx every trade you log carries its planned stop level and its actual exit price. The dashboard computes the R-multiple of every closed trade — `actual loss / planned risk`. A clean -1.0R loser is a respected stop. A -2.5R or -4R loser is the warning: "the stop wasn't honored."
When you scan the Journal tab at end of week, the stat that catches you is your average loss in R. If your average win is +1.6R and your average loss is -1.8R, you're not losing because of bad entries — you're losing because stops aren't holding. The fix is rarely strategy. It's almost always one of: hard stop instead of mental, accept slippage, never move the stop further away. TradeOnyx surfaces both numbers in the Overview tab so the math is impossible to look past.