Formally, in biosynthesis of terpenes, two or more isoprene molecules are linked to one another.  Linking between two isoprene molecules could occur in three ways, given that the head and the tail of the molecule are primarily involved in the linking:

1.  The head of one isoprene molecule could link with the head of another isoprene molecule.

This link is called a head-to-head or 1-1 link.

2.  The head of one isoprene molecule could link with the tail of another isoprene molecule.

This link is called a head-to-tail or 1-4 link.

3. The tail of one isoprene molecule could link with the tail of another isoprene molecule.

This link is called a tail-to-tail or 4-4 link.

Cyclic terpenes also contain links that are neither 1-1, 1-4, nor 4-4, which are called crosslinks.

Isoprene rule states that, in most naturally occurring terpenes, there are no 1-1 or 4-4 links.

eg. 1:

Myrcene

eg. 2:

Limonene

eg. 3:

Retinol

A terpene that does not obey the isoprene rule is called an irregular terpene.

eg:

β-Carotene

see also isoprene unit