Proofreading? Who needs it when you've got a spellchecker? We all have it, most of us use it, many of us swear by it: the spellchecker is the computer's modern-day answer to proofreading. But just how much should we trust this tool?

There's no doubt that the spellchecker can be useful in catching those random typos, saving you from 'embarasing errers' in your writing. But if you're going to use a spellchecker, it's essential to be aware of their limitations as well, or your writing will be far from perfect.

As a professional proofreader and copy editor, part of my job is to ensure my clients' material is written accurately. The vast majority of the time, the work I am passed has been spellchecked by the client beforehand, and yet I will still find countless mistakes in their material. Why?

1. Spellcheckers can tell you whether a word is correctly spelt, but not whether it is properly used. For example, your spellchecker will have no problem with the following: 'The bra was nosy and full off smock' (The bar was noisy and full of smoke) and 'You're diner is their' (Your dinner is there).

2. Most British computers come with an Americanised spellchecker, which will delight in telling you to change colour to color, and summarise to summarize.

3. Unless you change the settings, your spellchecker will automatically correct certain words as you type into what it assumes you meant. Helpful at times (e.g. if you just typed 'what'), but sometimes your computer may be right off the mark.

So, in practice your spellchecker will miss many, many mistakes. And if you think that's bad, watch out for the accompanying grammar checker. While it will helpfully point out the odd double space mid-sentence, it will completely miss gems like: 'The companies what make it programmes should big improve her grammar cheques.'

In view of these limitations, it's clear that while spellcheckers have their uses, it is foolish to rely on them entirely. In the 'good old days' people relied on two essential tools to ensure their writing was accurate: their dictionary and their eyes. I'm afraid there is no substitute for this age-old winning formula. By all means use your spellchecker, but carefully and with a degree of suspicion, and then follow it up with a slow, thorough proofread.