The smallest unit of length on a 300 mm ruler depends on the markings on the ruler. Here's how to figure it out:

* Standard Ruler: Most rulers have markings for millimeters (mm). In this case, the smallest unit is 1 mm.

* More Detailed Rulers: Some rulers might have markings for smaller units, such as:

* Half Millimeters (0.5 mm): The markings would be half the distance between the millimeter marks.

* Tenths of Millimeters (0.1 mm): The markings would be ten times more frequent than the millimeter marks.

To determine the smallest unit on your specific ruler, look at the markings between the millimeter lines.