Neckline

What Does Neckline Mean?
A level of support or resistance found on a head and shoulders pattern that is used by traders to determine strategic areas to place orders. Each peak of a regular head and shoulders pattern falls toward a support level, also known as a neckline, before it rises to create the next peak. A move below the neckline (in the case of a head and shoulders top) is used by traders as a signal of a reversal of the current uptrend.

Investopedia explains Neckline
It is possible to see the slope of a neckline be drawn at a slight angle rather than horizontal like the one shown in the image above. However, regardless of its slope, a move below the neckline of a regular head and shoulders pattern is always used as a signal of a move lower.
 
In the case of an inverse head and shoulders, the neckline is found as a level of resistance that has prevented the price from heading higher. A move beyond the neckline in this case would be used to signal the start of an uptrend.