If you read the Internal Revenue Code (and you probably don’t want to!), you may be surprised to find that most business deductions aren’t specifically listed. For example, the tax law doesn’t explicitly state that you can deduct office supplies and certain other expenses. Some expenses are detailed in the tax code, but the general rule is contained in the first sentence of §162, which states you can write off “all the ordinary and necessary expenses paid or incurred during the taxable year in carrying on any trade or business.” Basic definitions In general, an expense is ordinary if it’s considered common or customary in the particular trade or business. For example, insurance premiums to protect a store would be an ordinary business expense in the retail...