= Prenyl group
The specific five-carbon <prenyl group> is the <alkenyl group> 3-methylbut-2-en-1-yl, with connectivity
$$
\mathrm{R-CH_2-CH=C(CH_3)_2}.
$$
Here $\mathrm{R}$ is the rest of the attached <molecule>; the <prenyl group> itself has composition $\mathrm{C_5H_9}$. The ChEBI structural record uses SMILES `*CC=C(C)C`, with the asterisk marking the attachment point.<Prenyl group in ChEBI>[CHEBI:26248; formula, systematic name, SMILES and alkenyl classification]{ref}
The count is instructive: the attachment-end $\mathrm{CH_2}$ contributes one <carbon> and two <hydrogen> atoms; $\mathrm{CH}$ contributes one and one; the next double-bonded <carbon> contributes no <hydrogen>; and the two methyl groups contribute two <carbon> and six <hydrogen> atoms. Together these give five <carbon> and nine <hydrogen> atoms. The double bond is one bond away from the attachment atom, connecting the <prenyl group> with the broader <allyl group> pattern.
Attaching the group through oxygen gives $\mathrm{R-O-CH_2-CH=C(CH_3)_2}$, a prenyl <ether>; attachment through another atom gives a different linkage. The same carbon fragment therefore does not specify the whole <functional group> environment. In biological <prenylation>, the terminology can also encompass longer isoprenoid-derived attachments, so the particular transferred group must be stated.
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