In the chandas (Vedas) the root-vowel of tan and pat is elided before an affix beginning with a vowel, when it has an indicatory k or ṅ ,
As वितत्निरे कवयः, शकुना इव पप्तिमः ॥ In secular language वितेनिरे, पेतिम् ॥,
तनि-पत्योश् छन्दसि
Adhyāya 6 · Pāda 4 · Rule 99