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The rules are laid out in article 5 of the Schengen Visa Code:
- The Member State competent for examining and deciding on an application for a uniform visa shall be:
(a) the Member State whose territory constitutes the sole destination of the visit(s);
(b) if the visit includes more than one destination, the Member State whose territory constitutes the main destination of the visit(s) in terms of the length or purpose of stay; or
(c) if no main destination can be determined, the Member State whose external border the applicant intends to cross in order to enter the territory of the Member States.
Note that βmain destinationβ is defined based on the length or the purpose of the visit. So, for example, if you are going to attend a wedding in Rome, it will be pretty clear that Italy is your main destination.
If no main destination can be determined, it's still not up to you, you have to apply to the country you will enter first.