This principle simply stresses  that each tax should be levied at a time  or  in a way  in which it is most likely to be convenient  for the tax to pay e.g. a farmer should be taxed after harvesting his products.

The economy Principle. A good system  must be economic. Hence the cost of tax collection  must be as low as possible  so as to enable  the government raise  more revenue. The above were  canons according to Adam Smith. However other economists suggested  more principles as below:-

Elasticity Principle: A tax system should be capable of being altered to meet changing  fiscal requirements. i.e. it  should  be flexible in  order  to respond  automatically  to changes in the society's wealth. E.g. during a boom taxes  must be high and low  during a slump.

Equity Principles: Individuals should be taxed according to their taxable  capacities or ability to pay in relation to their incomes. It should also consider vertical and  horizontal equity.

Productivity Principles: A good   tax system  must not discourage production and the level of investments in the economy.

Simplicity Principle: A good tax system ought to be simple,  plain and easy to understand by tax payers and assessors, hence even  the  calculations should be simple too.

A good  tax system  must ensure proper allocation  of resources without having a disincentive  effect  on people's effort. E.g high taxes  may discourage overtime work  and may encourage migration to other countries with  lower taxes  on people's effort.

A good tax system  ought to be comprehensive so that it covers  most sectors of the economy in order to raise  more revenue for the government.

A tax system ought  not  to easily evaded or avoided.

Principle  of diversity: The tax system ought to levy different types of taxes so that the burden of these taxes  is on different groups of the society.

A tax system ought to minimize the possibility of double taxation i.e taxing a tax payer twice.