(2) Revenue

Revenue Recognition

Net sales include products and shipping and handling charges, net of estimates for product returns and any related sales incentives. Revenue is measured as the amount of consideration we expect to receive in exchange for transferring products. All revenue is recognized when we satisfy our performance obligations under the contract. We recognize revenue by transferring the promised products to the customer, with the majority of revenue recognized at the point in time the customer obtains control of the products. We recognize revenue for shipping and handling charges at the time the products are delivered to or picked up by the customer. We estimate product returns based on historical return rates. Using probability assessments, we estimate sales incentives expected to be paid over the term of the contract. The majority of our contracts have a single performance obligation and are short term in nature. Sales taxes and value added taxes in foreign jurisdictions that are collected from customers and remitted to governmental authorities are accounted for on a net basis and therefore are excluded from net sales.

Accounts Receivable

Credit is extended based upon an evaluation of the customer's financial condition. Accounts receivable are stated at their estimated net realizable value. The allowance for doubtful accounts is based on an analysis of customer accounts and our historical experience with accounts receivable write-offs.

Disaggregation of Revenue

Our revenues related to the following geographic areas were as follows for the periods ended June 30:



	Six-month Period				Three-month Period
	2018		2017		2018		2017
United States	$	2,121.1	$	1,908.4	$	1,092.6	986.1
Canada	151.8		123.3		79.2		63.7
Other foreign countries	180.8		137.5		96.1		71.7
Total revenues	$	2,453.7	$	2,169.2	$	1,267.9	1,121.5

The percentages of our sales by end market were as follows for the periods ended June 30:



	Six-month Period				Three-month Period
	2018		2017		2018		2017
Manufacturing	66.8	%	66.5	%	66.6	%	66.5	%
Non-Residential construction	12.9	%	13.0	%	13.2	%	13.0	%
Other	20.3	%	20.5	%	20.2	%	20.5	%
	100.0	%	100.0	%	100.0	%	100.0	%

The percentages of our sales by product line were as follows for the periods ended June 30⁽¹⁾:



		Six-month Period				Three-month Period
Type	Introduced	2018		2017		2018		2017
Fasteners⁽²⁾	1967	35.2	%	35.9	%	35.4	%	36.1	%
Tools	1993	10.0	%	10.1	%	9.9	%	9.9	%
Cutting tools	1996	5.7	%	5.8	%	5.6	%	5.8	%
Hydraulics & pneumatics	1996	6.9	%	6.9	%	6.9	%	6.8	%
Material handling	1996	5.8	%	6.0	%	5.9	%	6.0	%
Janitorial supplies	1996	7.5	%	7.2	%	7.6	%	7.2	%
Electrical supplies	1997	4.7	%	4.9	%	4.6	%	5.0	%
Welding supplies	1997	4.2	%	4.2	%	4.1	%	4.2	%
Safety supplies⁽³⁾	1999	16.9	%	16.0	%	16.9	%	16.0	%
Other		3.1	%	3.0	%	3.1	%	3.0	%
		100	%	100	%	100.0	%	100.0	%

⁽¹⁾In 2018, we reclassified certain product category designations and have conformed the prior period percentages to the current year presentation.

⁽²⁾Fastener product line represents fasteners and miscellaneous supplies.

⁽³⁾The safety supplies product line has expanded as a percentage of sales in the last several years due to our industrial vending program.