Dialogue has its origins in joint activities, which it serves to coordinate. Joint activities, in turn, usually emerge in hierarchically nested projects and subprojects. We propose that participants use dialogue to coordinate two kinds of transitions in these joint projects: vertical transitions, or entering and exiting joint projects; and horizontal transitions, or continuing within joint projects. The participants help signal these transitions with project markers, words such as uh-huh, m-hm, yeah, okay, or all right. These words have been studied mainly (...) as signals of listener feedback (back-channel signals) or turn-taking devices (acknowledgment tokens). We present evidence from several types of well-defined tasks that they are also part of a system of contrasts specialized for navigating joint projects. Uh-huh, m-hm and yeah are used for horizontal transitions, and okay and all right for vertical transitions. (shrink)