phyag rgya chen po - Mahamudra
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To understand the meaning, for example, if a king would decree an edict, he or she would use the great seal to stamp this, so people in the kingdom would know that it was originate from the true source.
There are many forms of mahamudra, of which one presentation is the emptiness of the mind and phenomena, this is what seals reality.
The phyag rgya chen po teachings mostly originated from the Kagyu tradition, and they are also present in the Gelug and Sakya traditions. bka' brgyud (Kagyu) is an interesting word, indeed, bka' means the words of the Buddha, and brgyud means transmission.