I recently started working my way through Hindemith's Traditional harmony Book 1 on my own. The instructions are short and simple, but sometimes too much so for a self-learner.
I have reached Chapter II; so far, the book is based on harmony for SATB voices. Hindemith starts this chapter with rules to connect the principal triads in their "simplest" form i.e. with the root tone doubled "in the octave or unison":
I am confused by this rule. Could someone clarify what is the reference point for the doubling interval? Is it relative to the Bass voice? The Tenor voice? (Other?) Please help me understand how it works.