Sketchnoting is a form of visual notetaking where people listen to, synthesize, and visualize ideas from a talk or other event in real time using a combination of pictures, diagrams and text. While becoming an increasingly used technique, only little is known about the higher-level strategies and techniques of this kind of visual note taking. We introduce a taxonomy of sketchnote styles and techniques, with a qualitative analysis of 103 sketchnotes posted on Twitter. The findings distil core sketchnote components: content, layout, structuring elements and visual styling. We synthesize dimensions of the sketchnote design space, classifying levels of conciseness, structure, illustration, unity, personification and realism. This leads to our discussion of design intents – common strategies to address particular goals or note taking challenges - which we situate in context of six semi-structured follow-up interviews with sketchnoters. We discuss how our taxonomy can inform the implementation of digital notetaking tools.