Market definition in multi-sided markets

Drawing from the economics of two-sided markets, I provide methodological suggestions for the definition of the relevant market in cases involving multi-sided platforms. In particular, I provide suggestions regarding a) how to identify the two-sided nature of a market; b) when multi-sidedness should be taken into account; c) how many markets should be defined; d) how the SSNIP or HM test should be performed; e) how the relevant market should be defined when one-side of the market is free. I also discuss when and to what extent one-sided methods may be harmless, or even useful.