##  Class files

This error appears when \ref{...} is used and LaTeX is unable to find the corresponding label. There are three ways in which this can happen. There is a typo in \label{...} There is a typo in \ref{...} A label has been forgotten. An example is shown below \section{introduction}\label{introduction} A typo when referencing the introduction would be \ref{intorduction}.   Another error is referencing a label which has never been defined such as \ref{section1} Each of these will generate an error which looks like main.tex, line 6 Reference intorduction' on page 1 undefined on input line 6. main.tex, line 6 Reference section1' on page 1 undefined on input line 8. main.tex, line 6 There were undefined references. To avoid this, we must always make sure that when we use \ref{...}, that there is a corresponding \label{...}, and that the arguments of \ref{...} and \label{...} are the same.