You shall not do wrong in judgment - in length, weight, or volume. Fair balances, fair weights, a fair efah, and a fair hin you shall have; I am Hashem your God, who brought you out of the land of Egypt. (19:35-36)
The first verse mentions three types of measurement in which we must not cheat people. The second verse gives examples, with the efah and hin being units of volume, or rather containers with volume equal those units.
The second verse follows the order of the first verse: weight measurement, then volume measurement. Length measurement, however, is conspicuously absent. Shouldn't a fair yardstick and tape measure have been mentioned before the fair balance, weights, efah, and hin?
Here are two possible explanations for why they are not mentioned.
1) Dishonest length measuring tools could be immediately detected by placing them next to fair tools, while with weight and volume measures the deception would be less obvious. Thus, perhaps, dishonesty was less common in length than in volume or weight measurements, so they did not need to be mentioned multiple times for emphasis.
2) Perhaps length was normally measured not with tools but with body parts. We are familiar with halachic measurements such as cubits and thumb-breadths, and secular measurements such as "feet" and paces, which derive from the human body. It is certainly possibly to cheat somebody in a length measurement, by bending a body part or changing your pace length. But this cheating was not normally done using a dishonest tool.
The first verse bans cheating. The second verse bans tools whose only possible use is unacceptable (like skeleton keys and "assault weapons" nowadays). Since body parts cannot be outlawed, they are omitted from the second verse.