It's pretty much implied, considering that everyone says that the sage was the only one with rinnegan..... Tobi/madara call themselves the 2nd So6P, and Nagato was the 3rd
The manga didn't make it clear what eyes the Elder son got.
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Look at the Younger son's picture. The fire around him is symbolic to show he has power in his body. It doesn't mean he was walking around on fire or anything.
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Look at the Elder son's picture. The swirls in his eyes may not show a particular dojutsu, they may be used to emphasise that he has power in his eyes. It doesn't necessarily mean that his dojutsu literally looked like that, just as the younger son didn't literally walk around of fire.
The manga didn't make it clear what eyes the Elder son got.
You must be registered for see images
Look at the Younger son's picture. The fire around him is symbolic to show he has power in his body. It doesn't mean he was walking around on fire or anything.
You must be registered for see images
Look at the Elder son's picture. The swirls in his eyes may not show a particular dojutsu, they may be used to emphasise that he has power in his eyes. It doesn't necessarily mean that his dojutsu literally looked like that, just as the younger son didn't literally walk around of fire.
the sage had 2 sons so his power got divided by two
by this, the original rinnegan changed its shape and became what it became in elder`s eyes
meaning, elder`s doujutsu = rinnegan - younger`s abilities AKA EMS
(which lated devolved to MS, 3T etc.)
when a person has EMS(elder`s power) and adds mokuton(younger`s power)
the process goes back, meaning, EMS+mokuton = rinnegan
the sage had 2 sons so his power got divided by two
by this, the original rinnegan changed its shape and became what it became in elder`s eyes
meaning, elder`s doujutsu = rinnegan - younger`s abilities AKA EMS
(which lated devolved to MS, 3T etc.)
when a person has EMS(elder`s power) and adds mokuton(younger`s power)
the process goes back, meaning, EMS+mokuton = rinnegan