According to the wikipedia

"The instrument's standard English name, double bass is derived from the tuning of one octave lower than the cello. It is not related to physical size. The name may derive from its alleged viol family heritage, in that it is tuned lower than the standard bass viola da gamba. The name also refers to the fact that the sounding pitch of the double bass is an octave below the bass clef."

 -------------- Original message ----------------------
From: Mike Miller <mwmiller@CWNET.COM>
> I was emailing with someone on the list who made string instruments and I
> forgot to ask the primary question:
>
> How come a stand up bass is called a double bass?
>
> Mike