If two or more than two forces are acting on a single point then the forces are known as system of concurrent forces and if they are acting on a single plane then the forces are called as coplanar forces. Copy the link given below and paste it in new browser window to get more information on Lami's Theorem:- http://www.transtutors.com/homework-help/mechanical-engineering/force-systems-and-analysis/lamis-theorem-solved-examples.aspx