linchpin /linchˈ or linshˈpin/

1. A pin used to keep a wheel on its axle
2. A person or thing essential to a plan, organization, etc (figurative)

• days are numbered
• day-scholar
• day-scholars
• day school
• day schools
• day shift
• day shifts
• day-sight
• day-sights
• daysman
• daysmen
• Days of Awe
• days off
• days of grace
• days of yore
• days out
• dayspring
• daysprings
• daystar
• daystars
• day surgery
• daytal
• daytale
• daytaler
• daytalers