In some cases the discrete (non-infinitesimal) arc elasticity is used instead. In other cases, such as modified duration in bond trading, a percentage change in output is divided by a unit (not percentage) change in input, yielding a semi-elasticity instead.
It is important to note that elasticity and slope are, for the most part, unrelated. Thus, when supply is represented linearly, regardless of the slope of the supply line, the coefficient of elasticity of any linear supply curve that passes through the origin is 1 (unit elastic).  The coefficient of elasticity of any linear supply curve that cuts the y -axis is greater than 1 (elastic), and the coefficient of elasticity of any linear supply curve that cuts the x -axis is less than 1 (inelastic). Likewise, for any given supply curve, it is likely that PES will vary along the curve. 
A fundamental building block of economic theory is the fact that increasing (or decreasing) the price of a commodity reduces (or increases) demand for that ...
Recall that we always ignore the negative sign when analyzing price elasticity, so PEoD is always positive.