Editor: How about "Shimo Tsukihime?" (つき姫 rhymes with 月姫, 下つき meaning something written below, like notes in the margin)
Nasu: Ah, okay, how should I explain this... (Laughs) Toward the end of 1999, while we were thinking up the plot, we were trying to decide how the females themselves would interact with one another. At that moment I was writing a scene between Arc and Ciel. And I was thinking that in this scene, I would have to end up making Arc the dominant one... (*18) But I'm glad I didn't go that far. (Laughs)
Takeuchi: That's the first time I've heard of this.
Nasu: Really?!
Takeuchi: Yeah.
Nasu: It's written in the plot notes, like "What if we did this here? This is Shimo-Tsukihime!" (Laughs)
Editor: Your usual graffiti?
Nasu: No, this was something done in a word processor, very seriously. I thought "maybe if I'm lucky he'll say yes...?"
Takeuchi: I completely forgot about all this. (Laughs)

*18
Here he's talking about Arc manifesting a male [] organ for Ciel... that would have been interesting.

Type-Moon Long Interview