What is the Plural of TARDIS?

I made two pairs of these socks. Color work is never going to be my favorite thing, but when one is surrounded by such ardent fans, something must be done. The pattern itself was a fun knit; I actually didn't mind churning out four of them.

Pattern: Tardis Socks Tribute by Ellen Botilda
Yarn: Jawoll Superwash Solids in Royal Blue, plus oddments of white and black

I've come a long way since having to have Daleks explained to me. I've even become a fan myself. In fact, there was a maple chocolate Dalek not long ago. But of course, he was destroyed. Rapidly. By forks, if I recall...