The sixteen-name mahamantra of Krishna and Rama — taught in the Kali-Santarana Upanishad as the foremost means of crossing the age of Kali, and spread worldwide through Chaitanya Mahaprabhu's sankirtana movement.
No restrictions of ritual purity, time or place apply — it may be chanted silently, spoken or sung. In Gaudiya Vaishnava practice, japa on a tulsi mala and congregational kirtan are both cherished forms.
Traditions read it both ways: as Ramachandra, and in Gaudiya interpretation also as Balarama or “Radha-ramana” (Krishna, beloved of Radha). The potency of the name is held to be the same.
Source: Kali-Santarana Upanishad. Regional versions may vary — this text follows the version most widely used in community worship.