[Tex/LaTex] Setting margins exactly

Question

My thesis formatting requires 1.5-inch margin on the left side and a 1-inch margin on the three remaining sides. Since all pages must be printed on one side of the page, I used the following:

\documentclass[oneside,12pt]{book}
\usepackage[top=1in, bottom=1in, left=1.5in, right=1in]{geometry}
\pagestyle{plain}
\begin{document}

some text here

\end{document}

But something is wrong. On the even pages the left margin is little smaller than 1.5 inch. And on the odd pages the right margin is less than 1 inch. I couldn't figure out what's wrong.

Answer 1

Normally, A4 papersize is assume, but I specified it directly. The showframe option to the geometry package shows the margins

\documentclass[oneside,12pt,letterpaper]{book}
\usepackage[letterpaper,top=1in, bottom=1in, left=1.5in, right=1in,showframe]{geometry}
\usepackage{blindtext}
\pagestyle{plain}
\begin{document}

\blindtext

\end{document}

Print this document, use a ruler and measure the margins with it.

The screen shot shows the frame, the thick black border is from the window background, it has nothing to do with the *.pdf output.