To stabilize the international tax system in a durable and sustainable way, three separate sets of profit allocation rules should be implemented, corresponding to the three substantially different firm structures that currently co-exist: multinational pipeline firms, centralized platform firms with global network effects and a unitary, albeit geographically distributed, user base, and decentralized platform firms with national, regional or local network effects and largely discrete user bases across taxing jurisdictions. The extant profit allocation rules work satisfactorily in relation to multinational pipeline firms, although they could be improved on significantly with certain comparatively limited modifications, as reviewed in Chap. 5.