Overview of characteristic length scales in the Vela X-1 system. Some of them are relatively well determined, on the order of 10%, and can be considered effectively fixed over the timescale of known observations. Other scales, shown with different types of hatching or shading, strongly depend on different parameters, as explained in the text. They may vary on short timescales of sometimes over an order of a magnitude or more, and a variation in one may drive changes in another. The ranges shown in this figure for these parameters are only to be taken as indicative. For comparison, the orbital separation corresponds to approximately 1/4 AU.