Darnley's behaviour was becoming so erratic he couldn't even differentiate between a work event and a party. So, Mary withheld from him the 'crown matrimonial' meaning Darnley was king in name only, as much as he might like to style himself as a leader he had no actual power - it was like being leader of the Scottish Conservative Party

https://youtu.be/2pHy0u3Qsrw