Thickness of Fluid Between Members refers to how resistant a fluid is to moving through it. For example, Water has a low or "thin" viscosity, while honey has a "thick" or high viscosity. And is denoted by t. Thickness of Fluid Between Members is usually measured using the Millimeter for Length. Note that the value of Thickness of Fluid Between Members is always negative. Typically, the value of Thickness of Fluid Between Members is greater than 0.