Newbie Question: The model equivilent of "helpers"

If I have ruby code that I need to share among a number of model
classes,
what’s the “right” place to put it form a Ruby standpoint?

The code provides support for various calculations. It has no data
elements
that are persisted in a database. It seems like there should be
something
like a helpers directory for models for this kind of supporting code.

thanks for your help.
-larry