this Special Isoprene Rule(SIR) was proposed by Ingold in 1925.
it states that" isoprene units in the terpenoid are linked together".
branched portion of isoprene unit is head and unbranched one is tail.
as per special isoprene rule the isoprene units are linked in head to tail fashion.
/ : /
head tail head tail
eg: myrcene, linonene, cadinene,abietic acid etc
exceptions to the SIR: cyrtone, lavandulol etc
cryptol has 9 C atoms and cannot be divided into two isoprene units.