Chunk format is used to ensure an appropriate balance between factual detail and the writer's thinking. Basically, a chunk is made up of one concrete detail (CD) followed by two commentary sentences (CM).

CONCRETE DETAILS
A concrete detail (CD) is generally a quote from the text or another individual, evidence or factual information, or a brief description of events that happened. It is important to note that CDs contain no opinion. Writers should exercise caution to avoid using words or phrases in a summative CD that reflect their personal views.

COMMENTARY
Commentary sentences initially begin with the phrases "This shows that..." and "This also shows..." Developing writers should use these phrases to check themselves - it is important to look at the commentary and ask if the conclusion being drawn really is shown in the text. As writers become more comfortable with the structure, phrasing can advance to include alternate verbs (this demonstrates, his comments illustrate, etc.).