diaeresis or dieresis /dī-ērˈi-sis or -erˈ/

1. A mark ( ¨ ) placed over a vowel, esp the second of two adjacent ones to show that it is to be pronounced separately, as in naïf
2. A pause or break where the end of the word coincides with the end of a foot (prosody)

• hardest
• hardface
• hard-faced
• hardfaces
• hard facts
• hard-favoured
• hard-favouredness
• hard-featured
• hard-featuredness
• hard feelings
• hard-fern
• hard-ferns
• hard-fisted
• hard-fought
• hard going
• hard-got
• hard-gotten
• hard-grained
• hardgrass
• hardgrasses
• hardhack
• hardhacks
• hard-handed
• hard hat
• hard hats