$\mathrm{N}-$glycosidic linkage: A nitrogenous base is linked to the pentose sugar through a $\mathrm{N}-$glycosidic linkage to form a nucleoside. Purine nucleosides have $1\text{'}-9$ glycosidic linkage (carbon $1\text{'}$of pentose sugar and $9\text{'}$ nitrogen of adenine/guanine). Pyrimidine nucleosides have$1\text{'}-1\text{'}$  linkage i.e., pentose sugar carbon $1\text{'}$ and $1\text{'}$ position of thymine/cytosine).