  Decentralization has been touted as the principal security advantage which
propelled blockchain systems at the forefront of developments in the financial
technology space. Its exact semantics nevertheless remain highly contested and
ambiguous, with proponents and critics disagreeing widely on the level of
decentralization offered by existing systems. To address this, we put forth a
systematization of the current landscape with respect to decentralization and
we derive a methodology that can help direct future research towards defining
and measuring decentralization. Our approach dissects blockchain systems into
multiple layers, or strata, each possibly encapsulating multiple categories,
and it enables a unified method for measuring decentralization in each one. Our
layers are (1) hardware, (2) software, (3) network, (4) consensus, (5)
economics ("tokenomics"), (6) client API, (7) governance, and (8) geography.
Armed with this stratification, we examine for each layer which pertinent
properties of distributed ledgers (safety, liveness, privacy, stability) can be
at risk due to centralization and in what way. We also introduce a practical
test, the "Minimum Decentralization Test" which can provide quick insights
about the decentralization state of a blockchain system. To demonstrate how our
stratified methodology can be used in practice, we apply it fully (layer by
layer) to Bitcoin, and we provide examples of systems which comprise one or
more "problematic" layers that cause them to fail the MDT. Our work highlights
the challenges in measuring and achieving decentralization, and suggests
various potential directions where future research is needed.
