The frame support is an extension of the single hydraulic props conventionally used underground, and the first type developed in modern self-advancing hydraulic powered supports. Shields were first introduced in a northern West Virginia mine in 1975. Its acceptance by the industry largely established longwall mining as a viable coal mining method. In a two-leg shield, both legs are connected between the canopy and base plates. The caving shield and the base are joined by two lemniscate bars on each side that have a total of four hinges. Research in the 1980s demonstrated that the two-leg lemniscate shield is the most desirable one. So, since 1990, all new shields purchased are the two-leg type and it is now the only type used in US longwall mining. As the technology to make a larger leg prop became reality, a two-leg shield could have a large load-bearing capacity by using large diameter leg props.