ruler now to measure length with the ruler you have to first look at the ruler carefully and examine the graduations or the tick marks on that ruler you have to examine and say okay every tick mark means this this ruler every tick mark that's written on this ruler is one centimeter the unit is right over here i know that this black line is greater than two but less than three so my smallest unit of measurement is the one centimeter so i have a lot of confidence that this line is 2 point something we don't know where it is there's no number written to tell us what the point is what the first decimal point is but we can estimate it so i'm going to say that this line is 2.5 centimeters now this number i got from the device this number i estimated it's called the estimated digit it's this number that you measure from the device you can get it directly from it this one you're estimating every one of these large graduations is one centimeter every one of these small graduations or tick marks is 0.1 centimeters so here i know that this line is greater than 2 less than 3 so i know it's going to be 2 point now on this ruler we can kind of see the tick marks right there right here so this line appears to be a little bit greater than 2.5 a little bit less than 2.6 so i can put 2.5 centimeters but now we have to have the estimated digit so here i'm going to say it's probably something like 2.5 5 centimeters this is my estimated digit and these two digits i got from the ruler i just read it off the device