Let's not forget something else that was not mentioned: these measurements may be taken around both inside and outside corners. In other words, corners may be ignored.
Start at a doorway, fireplace, etc., and measure along the wall-meets-floor line. You must hit a receptacle within 6 feet. If you hit a corner first, bend the ruler around the corner and keep going.
The next receptacle must be within 12 feet of the previous one, even around a corner, and the next one within 12 of that one, etc. The last one must be within 6 feet of the next door, chimney. etc.
I call this the 6-12-12-6 rule, and the similar requirements for countertops I call the 2-4-4-2 rule. If that doesn't explain it to anyone, then a picture is probably needed.
Usually, you do not end up with a full even measurement, and end up with the last one nearer the last opening, or nearer the previous receptacle, than need be. In that case, adjust them all for approximately even spacing.