Nor-

The "nor-" prefix also includes the elimination of a methylene bridge in a cyclic parent compound, followed by ring contraction.

(The prefix "homo-" which indicates the next higher member in a homologous series, is usually limited to noncyclic carbons).

Similarly Matthiessen and Foster called narcotine, which has three methoxy groups, "trimethyl nor-narcotine".

[7] "Since that time the meaning of the prefix has been generalized to denote the replacement of one or more methyl groups by H, or the disappearance of CH2 from a carbon chain".

[4] Woolman believed that "N ohne Radikal" was a German mnemonic and likely a backronym, rather than the real meaning of the prefix "nor".

[9] Originally, "nor" had an ambiguous meaning, as the term "normal" could also refer to the unbranched form in a series of isomers, for example as with alkanes, alkanols and some amino acids.

The IUPAC encourages that older trivial names, like norleucine and norvaline, not be used;[11] the use of the prefix for isomeric compounds was already discouraged in 1955 or earlier.

1867 publication about nor-compounds
Opianic acid demethylation
Removal of atoms from rings: cyclopamine , an example of a complex C-nor-D-homosteroid , where the C-12 atom is removed from the normally 6-membered central steroid ring (C-ring) and migrated into the normally 5-membered adjacent D-ring, during the course of biosynthesis, so swapping these ring sizes and markedly changing bioactivity . (As "nor-" indicates removal, "homo" indicates homologation (addition) of the removed carbon to other ring.) The cyclopamine steroid natural product is responsible for cyclopia in lambs whose mothers ingest it in corn lily.