@msh210 Usually, I wouldn't make such a minor edit. The reason I did it this time was that when I first read the title of that question it took me a bit to figure out what it meant. I've tried to make it more obvious to others.
and once I was there, I added some arbitrary capitalization :-)
Rambam (Hil. Melachim 4:4) states that a pilegesh (which, in his view, is permissible only to a king) enters into this status "without kesubah and without kiddushin; with yichud alone he acquires her and she becomes permitted to him."