A more thorough explanation:

BIRRETUM

A birretum is a type of cap or coif that was worn by judges and serjeants-at-law in Britain in the past.

Example 1: The judge put on his birretum before entering the courtroom.

Example 2: The serjeant-at-law wore a birretum as part of his official attire.

These examples illustrate how a birretum was a type of headwear worn by judges and serjeants-at-law in Britain. The birretum was a symbol of their authority and status, and was worn as part of their official attire when performing their duties.