Traditionally chanting practice or textual study was preceded with an invocatory passage to help forge a link between the chanters, what is about to be chanted and its purport, as well as setting a context for textual study.
Thus each area of Study that the teacher and student were about to venture into were preceded by an appropriate Dhyānam Ślokam, or set of verses that specifically linked the chanters with the study.
Therefore if the opening verses would differ according to whether the focus was Veda Chanting or Study, the Upaniṣat, the Bhagavad Gītā, the Yoga Sūtra, etc.
The Dhyānam Ślokam here were taught by T Krishnamacharya and is the long form transmitted to me by TKV Desikachar which includes two prayers relating to the Krishnamacharya family lineage and spiritual tradition.
View or Download Vedic Chant Opening Prayers Full version with Chant notations as a PDF (version 6.1 December 2013)