Final answer:
The black four-in-hand necktie is required to be worn by male service members when they are on leave and wearing civilian clothes. Soldiers on leave were required to wear uniforms so that MPs could easily spot military members, but some servicemen sought to evade these restrictions by joining underground clubs. Military authorities were especially vigilant in patrolling bars that catered to homosexual men.
Step-by-step explanation:
The black four-in-hand necktie is required to be worn by male service members when they are on leave and wearing civilian clothes, as stated in the provided information. When soldiers were on leave, they were required to wear uniforms so that military police could easily identify them. To avoid these restrictions, some servicemen joined underground clubs that rented civilian clothes and kept their uniforms secure until they returned to base. However, military authorities were especially vigilant in patrolling bars that catered to homosexual men.