Constitution (both express and implied). For example, section 99 forbids the Commonwealth from giving preference to any State or part of a State "by any law or regulation of trade, commerce, or revenue". And as discussed below, an implied guarantee of freedom of political communication has been held to limit the Commonwealth's power to regulate political discourse. The list of powers assigned to the Federal Parliament is quite similar to that assigned by the United States Constitution to the Congress, but is in some respects broader: for instance, it includes "astronomical and meteorological observations", marriage and divorce, and interstate industrial relations. The interpretation of similar heads of power – for instance the Trade and Commerce Power in Australia and the Commerce Clause in the US – has in some cases been different. The constitution also provides some opportunities for Federal-State co-operation: any State can "refer" a "matter" to the Commonwealth Parliament, and the Commonwealth Parliament can exercise, "at the request or with the concurrence of the Parliaments of all the States directly concerned", any power which, at the time of Federation, could be exercised only by the British Parliament. Parliamentary structures Representation in the House of Representatives is based on population and ‘original states’ have equal numbers in the Senate. The two houses are equal in power except for certain restrictions in financial matters. For example, the Senate may not amend a supply Bill, although as the Australian constitutional crisis of 1975 demonstrates, it may defer or refuse to pass such a Bill altogether; Bills to impose taxation or appropriate revenue may not originate in the Senate; and the Senate may not amend a Bill so as to increase taxation. Again, federalism is evident in the process of constitutional amendment, which requires that the Bill to amend the Constitution be approved by a majority of electors overall and a majority of electors in a majority of States (that is, four out of the six). Additionally, amendments "altering the limits" of a State or diminishing its proportional representation in Parliament require the approval of electors in that State. Parliamentary government It was assumed by the framers, in line with British and local colonial tradition, that the executive government would consist of Ministers who were members of Parliament and "responsible", that is, answerable, to it, and that the continued existence of the government would depend on it maintaining the confidence in the