n–butane and iso butane are a pair of

Both differ in their carbon skeleton.

${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_3 \mathrm{and} {\left({\mathrm{CH}}_3\right)}_3\mathrm{CH} \mathrm{differ} \mathrm{in} \mathrm{root} \mathrm{word} \mathrm{of} \mathrm{IUPAC} \mathrm{name}.$

→ The isomers which have same molecular formula but differ in carbon chain(root word of IUPAC name) are called chain isomers.