1. The Roman collar is a sign of priestly consecration to the Lord. As a wedding ring distinguishes husband and wife and symbolizes the union they enjoy, so the Roman collar identifies bishops and priests (and often deacons and seminarians) and manifests their proximity to the Divine Master by virtue of their free consent to the ordained ministry to which they have been (or may be) called.
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PS: I can state 100 reasons why the laity SHOULD NOT wear the clerical collar.
Number 1 reason.
1. It's called a CLERICAL collar. Only clerics wear it!