The Chicago Manual of Style offers 30-day free trial periods, so if a client requests you adhere to the rules in this style manual, you can try it out for free and see if you're going to need it long term before making the purchase. A one-year, one-person subscription to Chicago costs $35, and the same subscription to the AP Stylebook costs $26. With AP, you can also buy software add-ons like Styleguard for Word to check your Word documents for adherence to AP rules.
Keep in mind that if you are self-employed, you can usually deduct the costs of these manuals and software add-ons on your tax returns since they generally qualify under "business expenses." Other common business expenses include membership dues to professional organizations, trade or industry-related publication subscriptions, or equipment you need to do your work.
The right style manual for you depends on the type of writing you do. For example, AP is widely used by those in the news business, and Chicago is widely used by magazine and book authors. Technical and academic writers have other style guides they may be required to stick to. Owning copies (or online subscriptions) to these guides can't hurt, but if you're like a lot of freelancers, even an expenditure of $35 needs to be carefully considered first.
There are also online resources that can help you without your having to spring for a subscription. For example, AP vs. Chicago offers plenty of answers as well as comparisons between these two style manuals for common questions. If you're on a tight budget, you can probably start here and do just fine, but if you get a picky client, it may be time to get your own subscription to the client's manual of choice.
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