This is the law regarding Maryland tag display. I was stopped last year in Annapolis by the A A county Police. LEO was pretty cool, only gave me a warning. He also gave a paper with this on it describing the Maryland law.

§ 13-411.

(a) On a vehicle for which two registration plates are required, one plate shall be attached on the front and the other on the rear of the vehicle.

(b) On a vehicle for which one registration plate is required, the plate shall be attached on the:

(1) Front of the vehicle for a Class F (tractor) vehicle; and

(2) Rear of the vehicle for every other vehicle.

(c) At all times, each registration plate shall be:

(1) Maintained free from foreign materials, including registration plate covers as defined in § 13-411.1 of this subtitle, and in a condition to be clearly legible; and

(2) Securely fastened to the vehicle for which it is issued:

(i) In a horizontal position;

(ii) In a manner that prevents the plate from swinging; and

(iii) In a place and position to be clearly visible.
Hope this helps.