The hydro armrest offers 3 dimensional telescoping support.  Typically used on the dominant arm of the user, the hydro arm swings from left to right, extends in and out, and also has a tension adjustable spring action up and down.  If you are looking for very light support but full range of motion, this arm is a good choice.  The hydro arm can also be used on the non-dominant hand although it is less common to use it this way. Armrests are universal.