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I means that if you have a King Charles I and later on a Karl comes to the throne then he will be known and King Karl II.

How are you trying to put them in your mod? With a few more details I might be able to help
 
Thanks for replying :)

It works when I do it like this:
northern = {
graphical_culture = westerngfx

arrawni = {
graphical_culture = norsegfx

color = { 148 254 252 }

male_names = {
Arald
}
female_names = {
Arina
}

from_dynasty_prefix = "av "

male_patronym = "ssone"
female_patronym = "sditer"
prefix = no # The patronym is added as a suffix

# Chance of male children being named after their paternal or maternal grandfather, or their father. Sum must not exceed 100.
pat_grf_name_chance = 80
mat_grf_name_chance = 0
father_name_chance = 0

# Chance of female children being named after their paternal or maternal grandmother, or their mother. Sum must not exceed 100.
pat_grm_name_chance = 50
mat_grm_name_chance = 20
mother_name_chance = 0

modifier = default_culture_modifier
}

tremuni = {
graphical_culture = norsegfx

color = { 69 129 121 }

male_names = {

}
female_names = {

}

from_dynasty_prefix = "i "

# Chance of male children being named after their paternal or maternal grandfather, or their father. Sum must not exceed 100.
pat_grf_name_chance = 80
mat_grf_name_chance = 0
father_name_chance = 0

# Chance of female children being named after their paternal or maternal grandmother, or their mother. Sum must not exceed 100.
pat_grm_name_chance = 50
mat_grm_name_chance = 20
mother_name_chance = 0

modifier = default_culture_modifier
}
}

central = {
graphical_culture = westerngfx

deep_low = {
graphical_culture = germangfx

color = { 149 139 72 }

male_names = {
Antius Antaeus Argos Argaeus Calderon Elidyr Ferron Ferraeus Kennrick Kennraeus Margeron Sif
}
female_names = {
Aura Aurea Arga Argea Elidyr Ferrea Kennrea Margarite
}

from_dynasty_prefix = "of "

# Chance of male children being named after their paternal or maternal grandfather, or their father. Sum must not exceed 100.
pat_grf_name_chance = 30
mat_grf_name_chance = 0
father_name_chance = 60

# Chance of female children being named after their paternal or maternal grandmother, or their mother. Sum must not exceed 100.
pat_grm_name_chance = 20
mat_grm_name_chance = 20
mother_name_chance = 10

modifier = default_culture_modifier
}

deep_high = {
graphical_culture = germangfx

color = { 79 112 54 }

male_names = {
Antius Antaeus Argos Argaeus Calderon Elidyr Ferron Ferraeus Kennrick Kennraeus Margeron Sif
}
female_names = {
Aura Aurea Arga Argea Elidyr Ferrea Kennrea Margarite
}

from_dynasty_prefix = "of "

# Chance of male children being named after their paternal or maternal grandfather, or their father. Sum must not exceed 100.
pat_grf_name_chance = 30
mat_grf_name_chance = 0
father_name_chance = 60

# Chance of female children being named after their paternal or maternal grandmother, or their mother. Sum must not exceed 100.
pat_grm_name_chance = 20
mat_grm_name_chance = 20
mother_name_chance = 10

modifier = default_culture_modifier
}
}

And then it doesn't when I put it like this:
northern = {
graphical_culture = westerngfx

arrawni = {
graphical_culture = norsegfx

color = { 148 254 252 }

male_names = {
Arald
}
female_names = {
Arina
}

from_dynasty_prefix = "av "

male_patronym = "ssone"
female_patronym = "sditer"
prefix = no # The patronym is added as a suffix

# Chance of male children being named after their paternal or maternal grandfather, or their father. Sum must not exceed 100.
pat_grf_name_chance = 80
mat_grf_name_chance = 0
father_name_chance = 0

# Chance of female children being named after their paternal or maternal grandmother, or their mother. Sum must not exceed 100.
pat_grm_name_chance = 50
mat_grm_name_chance = 20
mother_name_chance = 0

modifier = default_culture_modifier
}

tremuni = {
graphical_culture = norsegfx

color = { 69 129 121 }

male_names = {

}
female_names = {

}

from_dynasty_prefix = "i "

# Chance of male children being named after their paternal or maternal grandfather, or their father. Sum must not exceed 100.
pat_grf_name_chance = 80
mat_grf_name_chance = 0
father_name_chance = 0

# Chance of female children being named after their paternal or maternal grandmother, or their mother. Sum must not exceed 100.
pat_grm_name_chance = 50
mat_grm_name_chance = 20
mother_name_chance = 0

modifier = default_culture_modifier
}
}

central = {
graphical_culture = westerngfx

deep_low = {
graphical_culture = germangfx

color = { 149 139 72 }

male_names = {
Antius Antaeus Argos Argaeus Calderon Elidyr Ferron Ferraeus Kennrick Kennraeus Margeron Sif
}
female_names = {
Aura Aurea Arga Argea Elidyr Ferrea Kennrea Margarite
}

from_dynasty_prefix = "of "

# Chance of male children being named after their paternal or maternal grandfather, or their father. Sum must not exceed 100.
pat_grf_name_chance = 30
mat_grf_name_chance = 0
father_name_chance = 60

# Chance of female children being named after their paternal or maternal grandmother, or their mother. Sum must not exceed 100.
pat_grm_name_chance = 20
mat_grm_name_chance = 20
mother_name_chance = 10

modifier = default_culture_modifier
}

deep_high = {
graphical_culture = germangfx

color = { 79 112 54 }

male_names = {
Antius_Antaeus Argos_Argaeus Calderon Elidyr Ferron_Ferraeus Kennrick_Kennraeus Margeron Sif
}
female_names = {
Aura_Aurea Arga_Argea Elidyr Ferra_Ferrea Kennra_Kennrea Margarite
}

from_dynasty_prefix = "of "

# Chance of male children being named after their paternal or maternal grandfather, or their father. Sum must not exceed 100.
pat_grf_name_chance = 30
mat_grf_name_chance = 0
father_name_chance = 60

# Chance of female children being named after their paternal or maternal grandmother, or their mother. Sum must not exceed 100.
pat_grm_name_chance = 20
mat_grm_name_chance = 20
mother_name_chance = 10

modifier = default_culture_modifier
}
}
 
Not sure how the whole process works but I think you need to have a common name to link all the culture names, yours is quite muddled at the moment. Try listing all the names you want for the first culture then use them as the common name which you link to other culture names with _. e.g

English

Charles

Spanish

Carlos_Charles

Danish

Karl_Charles

etc.

Put the culture name first then the common name after.

This is all speculation though.
 
I did some searching in the files for "Matthew" (which is one of those paired names). Two things strike the eye:

1) There are entries "Matthew_Matthew"
2) It always appears as the second partner (f.e. "Matthieu_Matthew") as far as I can tell

It seems Matthew then functions as a defining group name to the family of Matthew-ishes. So that means I need to add entries like "Antaeus_Antaeus".
Will test that now.

EDIT: It works! Apparently me and SirDraco were right. Something to put in a modding guide somewhere?
 
Apparently these name similarities also have an effect in child education.
In a playthrough as the King of Ireland, I had my daughter Caitríona educated by a german courtier. When she matured at 16, not only her culture had shifted to german (which is no surprise), but she also changed her name to Katharina (which would be a rough german translation).