The four words in the first set below share something with the four words in the following set. Can you determine what it is?

READ

ROUGH

UNDER

RUSH

STEAL

FOUR

HEAL

DEAR

HINT #1: It has to do with the same two letters.

HINT #2: The same two letters can precede the words in the first set and follow those in the second.

Answer:

THREAD

THROUGH

THUNDER

THRUSH

STEALTH

FOURTH

HEALTH

DEARTH