plot xaxp issue

Hello,

I figured out that the code should be

boyline<-lm(body_weight ~ body_length, data=subset (together,,sex=="boy"))

However, the "" could be omitted if the field name happened to be
numeric, such as 1, 2, or 3.
Please kindly explain why the "" could be omitted for numbers.
Thanks again.

Elaine

boyline<-lm(body_weight ~ body_length, data=together, subset(together,sex=="boy"))