Haven't been around in a while, I was just checking back to the units threads and saw a bit more discussion 'bout adding temperatures, date and time, absolute vs. relative quantities. Now I suddenly saw the light: "absolute" is a relative term... 0.5f * pun_intended(). I will prove this once and for all time with a simple example of a mechanical drawing. If you've been procrastinating setting a monospaced font for viewing messages, procrastinate no more: ____________ V0 = 000 ("datum" line) | __ | / \ | | | -.- V1 = 150 | \__/ | | __ | / \ | | | -.- V2 = 450 | \__/ | | __ | / \ | | | -.- V3 = 750 | \__/ |____________ V4 = 900 (dimensions are in mils (thousands of an inch) ) assert( V4 - V3 == V1 - V0 ); // no problemo assert( V3 - V2 == V2 - V1 ); // no problemo assert( V1 + V2 + V3 >= V4 ); // uhhhh....??? In other words, dimensions that are all measured to the same edge are "absolute" relative to each other. (See I'm no punster?) So far so good. Now, what happens if I have... a = V2 - V1; b = V3 - V1; //NOT V3 - V2, note... Both we might call "relative", since they are both differences between absolutes, right? I should be able to add them right? WRONG!!! assert( a + b >= b ); // DOH!... a and b are both measured from V1, and therefore, relative to each other, they are, again, you guessed it, "absolute". In other words, forget the whole subject. I regret I brought it up in the first place. Leave it for the PhQS (Philosophical Quantities Library). Cheers!