Originally more '+' shaped with a longer lateral line than the character for ten, to represent and mean a line cuttting another. It was one of serveral characters used phonetically to express seven, and was probably especially favored since it roughly resembled a bent finger under a fist, an old way of signalling seven.
Read as either SHICHI or nana as in: shichigatsu (July), nanoka (seventh day), & nanaban (seven nights).