Pho Tek wrote: If I buy a simple plastic ruler from a book store, how do I know what precision I have at my disposal ?
The number of digits is controlled by two things: the tool used to measure, and the skill of the person making the measurement.
If the ruler has markings for each millimeter, then the finest measurement will be in millimeters. If you are measuring a distance of 1.234 meters, you have three significant digits. (it had better be a long ruler).
Its more complicated, as a really cheap ruler may not actually measure in meters/millimeters, but some error number (assume it is 50% off) then you will write done a lot of significant digits from your measurement, but few (none?) are accurate.
Someone with a good rule, good eyes, and skill may be able to see that rather than being about ten millimeters long, its really a bit longer, and so they can write down 10.2 milimeters. This means that there are two significant digits. The "2" may be wrong, but its much more likely to be 1 or 3, and its unlikely to be 10.7