The same two-letter combination can be put into the center of each of the following words to form a six-letter word. For example, you could put "sp" in the center of 'veer' to form vesper, but it won't work for the other words. What is the only two-letter combination that works for all six cases?

1. fist
2. host
3. liar
4. pals
5. veer
6. wiry