It is an unplanned product of slow historical growth, and is not in the form of a systematic and coherent document. The best example of an unwritten constitution is the British Constitution which was not made by any constituent assembly or any other authorised body of persons at any particular time. It has grown by degrees.
The distinction between written and unwritten constitutions should not be taken in a very literal sense. The difference between the two is one of degree rather than of kind. No constitution is wholly written or wholly unwritten. All written constitutions, even of a short duration, do contain unwritten elements in the form of usages and conventions.
And the so-called unwritten constitutions do contain some written provisions. The British Constitution, for example, contains important written elements such as Magna Carta of 1215 (it laid the basis of parliamentary power).
The Bill of Rights of 1689 (it deprived the monarch of lawmaking and taxing powers), the Parliament Acts of 1911 and 1949 (these Acts curtailed the powers of the House of Lords), and so forth.
Constitutions are also classified in terms of the ease or difficulty with which they can be formally amended. If a constitution can be amended by the legislature in the same way as ordinary laws are passed, it is said to be flexible the British Constitution, for example. The British Parliament can amend the constitution in the same manner in which it passes or repeals ordinary laws.
On the other hand, if the procedure of amending a constitution is different from that of passing or repealing ordinary laws, the constitution is said to be rigid. The USA, for example, has a rigid constitution. Congress (i.e., the US legislature) cannot amend it on its own. In the USA, every amendment proposed by Congress has to be ratified by the legislatures of at least three-fourths of the States (i.e., the federating units).
Constitution can also be categorised as unitary and federal ones. A constitution is said to be unitary if it concentrates all authority at the Centre, making the various regions (called provinces, states, etc.) subservient to the Central Government. Britain and France have unitary constitutions.
On the other hand, a federal constitution divides government authority between the Centre and the regions (i.e., provinces or states) conferring on the latter an autonomous status. Thus, the provinces or ‘States’ are not at the mercy of the Central Government for everything. The US Constitution, the Canadian Constitution and the Indian Constitution are examples of federal constitutions.