1. The problem statement, all variables and given/known data from the equation r = n+3 , some notes stated n = number of hinges , while some book stated n as number of joints ...Which is true ? 2. Relevant equations 3. The attempt at a solution taking the example 3.1 , if it's number of joints , then there are 1 hinge (pin) , right ? I dont really understand the notes on the internet ... What does joints in beam mean ?