Motivated by a characterization of weakly compact cardinals due to Todorcevic, we introduce a new cardinal characteristic, the C-sequence number, which can be seen as a measure of the compactness of a regular uncountable cardinal. We prove a number of ZFC and independence results about the C-sequence number and its relationship with large cardinals, stationary reflection, and square principles. We then introduce and study the more general C-sequence spectrum and uncover some tight connections between the C-sequence spectrum and the strong coloring principle U(. . .), introduced in Part I of this series. Contents 1. Introduction 1.1. Organization of this paper 1.2. Notation and conventions 2. The C-sequence number 3. Changing the value of the C-sequence number 3.1. Inaccessibles 3.2. Successors of singulars 4. The C-sequence spectrum 5. The C-sequence spectrum and closed colorings 5.1. From C-sequences to closed colorings 5.2. From closed colorings to C-sequences 5.3. From colorings to closed colorings 5.4. The structure of the C-sequence spectrum 6. Concluding remarks