# What does the letter R represent in chemical structures?

The letter $\ce{R}$ appears frequently in chemical structure illustrations. For instance, when I Google keratin, I find Wikipedia's Spanish article about it.

In the image illustrating part of the chemical structure, I see elements like $\ce{H, C, O, N, and S}$ - which I recognize - but I also see $\ce{R}$, which I do not. What does $\ce{R}$ stand for in this context?

• – Loong Aug 4 '17 at 8:37
• I'm wondering if we could make this more general so that it would be googleable. I'm sure it's a common question. – jonsca Aug 4 '17 at 11:12

• I actually thought that $\ce{R}$ originates from German "der Rest" (English "residue"), and the word "radical" was just an adaptation. I wonder if native German speakers could comment on this:) – andselisk Aug 4 '17 at 7:10