# Changing book chapters

How do i change book chapters from CHAPTER 1 to CHAPTER ONE. I have already tried these commands

\usepackage{fmtcount} % for textual representation of numbers
\renewcommand{\chaptermark}[1]{\markboth{\chaptername\ \thechapter.\ #1}{}}


You need \numberstring{chapter} or \Numberstring{chapter} in place of \thechapter. Here is a MWE. (Note that your code only change the style of the chapter number in the headings.)

\documentclass{book}
\usepackage{lipsum}
\usepackage{fmtcount}

\renewcommand{\chaptermark}[1]{\markboth{\chaptername\ \numberstring{chapter}.\ #1}{}}

\begin{document}

\chapter{Title}
\lipsum

\end{document}


The memoir class (a superset of book and report) has facilities for this. It provides several methods for printing numbers as words (27 = Twenty-Seven) or as ordinals (27 = Twenty-Seventh).

% chapnumprob.tex  SE 593655
\documentclass{memoir}
\usepackage{lipsum}

% Use named numbers for chapters
\let\savechapnum\thechapter
\renewcommand{\thechapter}{\NumToName{\savechapnum}}
% extra space for named chapter numbers in the ToC
\setlength{\cftchapternumwidth}{3em}
\begin{document}
% Uncomment the following line to get chapter arabic numbers in the ToC
\tableofcontents
\chapter{Title}
\lipsum

\setcounter{chapter}{26}
\chapter{Long number}
\lipsum

\end{document}


There is a general problem regarding the ToC if you use words instead of numbers for the chapter numerations. Note that by default the ToC prints chapter numbers as words so you have to allow enough space for the longest number words. On the other hand you could use words in the body of the document but numerals in the ToC as indicated in the above MWE.

I have not bothered to check what the result is if you

\chapter{A chapter} \label{chap}
In chapter\ref{chap} ...