The Crown of Feet
Śrī Śaṭhakopam (commonly Sadagopam, a respectful variant of Nammāzhvār's name Śaṭhakōpa) refers to the crown-like ornament shaped as the lotus feet of Nammāzhvār, placed on the heads of devotees at temples as the final and supreme blessing (prasāda). Receiving the sadagopam on one's head is the culminating act of temple worship — the devotee becomes the bearer of the Āzhvār's feet.
Why Nammāzhvār's Feet?
In the tradition, Nammāzhvār is the Āzhvār's Āzhvār — the supreme among the twelve. His Tiruvāymozhi is considered the Tamil Veda, and his experience of Bhagavān the fullest among all Āzhvārs. The placement of his feet-crown on the devotee's head transmits his grace directly — a physical enactment of āzhvār abhimānam.
At Āzhvārtirunagari
The original Śrī Śaṭhakopa Sadagopam is preserved at Āzhvārtirunagari (Nammāzhvār's birthplace). The Śrī Rāmānujam at Āzhvārtirunagari and the Ānanda-āzhvān at Tirupati also refer to Nammāzhvār's sacred feet as they are manifested at those specific temples.