Decimeter (dm): The decimeter is equal to one-tenth of a meter (0.1 meters). It is used less frequently than other metric units but can be encountered in certain applications.
The term "long reed" does not refer to a standard unit of measurement or a widely recognized term in contemporary systems of measurement. It's possible that "long reed" is used in a specific context or industry, but it does not have a standard definition in general measurements.
If you have more context or specific information about how "long reed" is used, I would be happy to try to provide a more precise explanation.