Before we discuss the fifth dimension let's introduce the other four. Building Information Modeling (BIM) can best be described as ontology (i.e. virtual reality). So just as in real life there are 3 dimensions of space (up and down, side-to-side, and back and forth) so there are in the virtual world. This is what makes up the extent of a project, how big or small it is and coupled with this is also the quality of the project. Next there is the 4th dimension of time, which includes managing the logistics of material and personnel, sequencing, and means and methods of construction. This brings us to the 5th dimension, cost. Tying the budget to the model allows for checking on costs throughout the design process. In construction it is widely understood that these three factors, extent/quality, time and cost, must be carefully considered and if more weight is placed on one it is not possible to have the others in equilibrium. For instance if primary emphasis is on budget and schedule then either the extent of the project must be minimized or the quality level decreased or both. 5D BIM manages these factors to aid in early decision making when the ability to impact cost and time is the greatest and the cost of changes is the least. This is part of an approach know as Integrated Project Delivery (IPD).