Best type of pen/ink for addressing invites?

I'm going to hand-address my invitations. I bought a template on for keeping the lines straight. For the people who hand addressed envelopes, what type of pen did

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FanFlex: Lightweight flexible crests and patches that fold easy. - FanTex: An all-new super comfortable soft fabric. - FanFit: The authentic on-ice

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FanFlex: Lightweight flexible crests and patches that fold easy. - FanTex: An all-new super comfortable soft fabric. - FanFit: The authentic on-ice

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