The last of the main sections of a city directory is the Classified Business Directory. As the title implies this section pertains only to businesses not to individuals. Using my R.L. Polk City Directory for Detroit, 1925-1926 as a reference, the Classified Business Directory section starts on p. 2465 and runs through p. 2648. The title page reads: Polk's, Detroit City Directory, 1925-1926, Classified Business Directory, "Names Appearing under Headings Marked Thus (*) Are Only Inserted When Specially Contracted For" (I take that to mean that there was no fee charged to be listed under general subheadings but if a specialty subheading was requested there was a fee charged for that listing).
The first thing you'll notice when perusing this section is that businesses are listed alphabetically under category subheadings (which are also listed alphabetically). Examples of general category subheadings include: Accountants, Bakers and Confectioners, and Cigars and Tobacco-Retail. Examples of "specially contracted for" listings include: Accountants-Certified Public, Bakers' Machinery, and Cigar Box Mfrs. For the most part, the specialty listings are more specific categories than the general categories.
The individual listings basically just give the name of the business, the street address (and building name if applicable), and in many but not all instances a telephone number. The names of some businesses are in bold print and these are the ones with phone numbers. To be honest I don't know if one had to pay extra to get a bolded listing with a phone number or if telephones were still new and uncommon enough that the publisher simply highlighted them as a convenience to the reader. I checked the listing for my grandfather's bakery and it did not appear in bold print. I know from his business card that he did have a phone at his business but I don't know if he had it by 1926 when my copy of the city directory would have been published. It did however list both locations of his business. In fact, this was how I first learned that my grandfather's business had a second location. No one in the family had ever mentioned that to me.
So while the information in this section is rather sparse (doesn't list business executives, owners, or the like), it's still a good resource for looking up a family business. It's also interesting to note which sorts of business were more popular than others. Some categories have only a handful of entries while others have a whole page or more. The subheading of "Contractor" for instance covers 7+ pages. During the 1920s, when this directory was published, there was a big influx of immigrants to the city and housing was in short supply. This is certainly reflected in the number of contractors doing business in the city at that time.
Coming up next is the final installment in this series: City Directories: Additional Information
Read my series of articles about city directories:
I Won the eBay Bid
What's In A City Directory
City Directories: The Introduction
City Directories: The Indexes
City Directories: The Statistical Department
City Directories: Chronological History
City Directories: Miscellaneous Information
City Directories: Directory of Names
City Directories: Street Guide and Directory of Householders
City Directories: Classified Business Directory
City Directories: Additional Information