Certain documents say your birth certificates, incorporation documents, passport copy or marksheets originating from one country, cannot just simply valid and acceptable in another country. For this, you would be asked to obtain legalisation or apostille on the documents. Only then it is considered valid. Note that simple notarisations is not acceptable in those countries where the documents are used.
Legalisation is a time consuming process and you have to follow a chain of procedures step by step. To minimise the hardwork involved, countries formed a network and signed an agreement called HAGUE AGREEMENT OF 1961 ABOLISHING THE REQUIREMENT FOR PUBLIC DOCUMENTS. Together they came up to issue a certificate called 'APOSTILLE' or 'APOSTIL' which is automatically considered valid and acceptable in within all hague members. Some countries, though may not be a member of hague agreement, very well recognise and accept apostilles. Thus, Apostille is a simplified Legalisation |