In Czech pronunciation there are pairs of consonants, one consonant of the pair is voiced, one is unvoiced:
ch-h (this pair has actually three members, but I don't want to overwhelm you)
There is also voiced and unvoiced "ř."
There are more changes in consonants of different origin, but for a basic conclusion of this particular one, you need to remember, that if you have a pair-consonant in the surname (or name of the village) and you struggle to find something, it could be simply switched to the other consonant of the same pair.
It's also important for using old indexes, as the surname may therefore occur under a different letter or the pair-consonants may be grouped together.