syn- /sin-/, sy- /si-/, syl- /sil-/, sym- /sim-/ or sys- /sis-/

Signifying together, with

• heave to
• heavier
• heavier-than-air
• heavies
• heaviest
• heavily
• heaviness
• heavinesses
• heaving
• heavings
• Heaviside layer
• heavy
• heavy-armed
• heavy breather
• heavy breathers
• heavy breathing
• heavy chemicals
• heavy cream
• heavy-duty
• heavy going
• heavy-handed
• heavy-headed
• heavy-hearted
• heavy horse
• heavy horses