In Vergilium Commentarius
Maurus (Marius) Servius Honoratus
Quos Romani penitus in cultu habent, quos nisi sacerdoti videre fas nulli sit, qui ideo penates appellantur, quod in penetralibus aedium coli soleant; nam et ipsum penetral penus dicitur, ut hodie quoque penus Vestae claudi vel aperiri dicitur.
Those (gods) which the Romans worship deeply, which it is right for no one except the priest to see, these are thus called the Penates [that is, protectors of the storehouse], because they are worshipped in the inmost parts of temples; for also the storehouse is called the inner sanctum, as today also the storehouse of Vesta is said to be closed or opened.
[Note: It is not possible to capture in English all the play in this passage on the word penus (storehouse) and its derivatives.]
Translation by Jane W. Crawford, © 2001.