The upside-down head-and-shoulders. A low, a lower low, then a higher low, with the recovery highs between them forming the neckline. Completion requires a close above that line.
The conventional target projects the distance from the head to the neckline upward from the break, a measured-move. Stops usually sit under the right shoulder, which gives a defined risk that is often much smaller than the target, and that geometry is the main reason the pattern is popular.
As with all named patterns, failure is frequent and the pattern is easiest to see once it has already worked. Requiring a retest of the neckline before entering trades a lower fill for fewer false starts, at the cost of missing the fastest examples.
Related: head-and-shoulders, neckline, measured-move, reversal-pattern, rounding-bottom