During the 1880s the states of Chamba, Faridkot, Gwalior, Jind, Nabha & Patiala agreed postal conventions with the Imperial Government by which the British Indian Post Office supplied stamps overprinted with the name of the state.
These states agreed to conform with stamp issuing policy, postage rates and to the handling of mail from other areas. Mail bearing these stamps could be delivered anywhere within British India.
Many collectors who might otherwise be wary of collecting the often primitive Feudatory State stamps enjoy building collections of Convention State issues as they are the same stamps as British India but simply with an additional overprint. Most of the listed errors on these stamps involve the overprints which can include errors of spelling, inverted, doubled, missing characters etc.