I find the references section on Wikipedia to be a good source of information. On the Helvetica Wiki page, I found a link to a great site called I Love Typography, which has a page called Arial versus Helvetica.
I Love Typography has something called a ‘Fontometer’ which compares fonts for you.
I used the fontometer to compare the character ‘a’ in Arial and Helvetica:
On the left is Arial, and on the right is Helvetica.
When placed side by side, you can only tell the slight differences, but when placed together, you can see in detail the exact diffrences in the characters:
The fontometer allows you to do this to every letter of the alphabet.
Seeing this has made me realise that they are actually quite similar, and I can certainly see why they are often compared.
I think that Arial is often used as the alternative to Helvetica, because it is the most similar font which is provided with Microsoft software. It’s a free, similar font.
