Rendaku

Rendaku (連濁^?, lit. "sequential voicing") is a phenomenon in Japanese morphophonology that governs the voicing of the initial consonant of the non-initial portion of a compound or prefixed word. In modern Japanese, 
Rendaku can be seen in the following:

[hito] + [hito] > [hitobito] ("person" + "person" → "people")

[toki] + [toki] > [tokidoki] ("time" + "time" → "sometimes")

[te] + [kami] > [tegami] ("hand" + "paper" → "letter")

In some cases, rendaku varies depending on syntax. For instance, the suffix 〜通り ("road, following"), from 通る tōru "to go, to follow", is pronounced as 〜とおり -tōri following the perfective verb tense, as in 思った通り omotta-tōri "as I thought", but is pronounced as 〜どおり -dōri (with rendaku) when following a noun, as in 予定通り yotei-dōri "as planned, according to schedule" (or, semantically differently – more concretely – 室町通 Muromachi-dōri, "Muromachi Street").