Use of the terms Primary, Secondary, and Teriary.

When applied to atoms or groups, other than to nitrogen, the terms primary, secondary, and tertiary refer to the carbon atom, or the carbon atom to which the referred-to atom or group is attached.

• Primary indicates that the carbon is attached to only one other carbon (or to none as in methyl).
• Secondary indicates that the carbon is attached to two other carbons.
• Tertiary indicates that the carbon is attached to three other carbons.
• Quaternary indicates a carbon attached to four other carbons.

In the following models, the atoms refered to have been coloured green:

Butan-1-ol (n-Butyl Alcohol)

For the compound butanol, CH3CH2CH2CH2OH:
There are two primary carbon atoms:
There are two secondary carbon atoms:
There are five primary hydrogens (because they are attached to the two primary carbons):
There are four secondary hydrogens (because they are attached to the secondary carbons):
The OH group is primary (because it is attached to a primary carbon):

Butan-2-ol (sec-Butyl Alcohol)

For butan-2-ol, CH3CH2CHOHCH3:
There are two primary carbon atoms:
There are two secondary carbon atoms:
There are six primary hydrogens (because they are attached to the two primary carbons):
There are three secondary hydrogens (because they are attached to the secondary carbons):
The OH group is secondary (because it is attached to a secondary carbon):

2-Methylpropan-2-ol (t-Butyl Alcohol)

Finally, for 2-methylpropan-2-ol, (CH3)3COH:
There are three primary carbon atoms:
There is one tertiary carbon atom
There are nine primary hydrogen atoms:
The OH group is tertiary:

Important exception : nitrogen

Primary, secondary, tertiary, and quaternary in nitrogen containing compounds refer to the number of carbon atoms attached to the nitrogen atom!