Revenue and Accounts Receivable, Net

Revenue Details

The pattern of revenue recognition was as follows:



	Year Ended December 31,
(In thousands)	2019		2018
Goods and services transferred at a point in time	$	558,361	$	577,085
Goods and services transferred over time	195,116		103,156
Net sales	$	753,477	$	680,241

Our net sales to external customers by geographic area were as follows:

Specific identification of the customer billing location was the basis used for attributing revenues from external customers to geographic areas.

Net sales to the following customers totaled more than 10% of our net sales:

⁽¹⁾Sales associated with this customer did not total more than 10% of our net sales for the indicated period.

Accounts Receivable, Net

Accounts receivable, net were as follows:



	December 31,
(In thousands)	2019			2018
Trade receivables, gross	$	130,888		$	133,774
Allowance for doubtful accounts	(1,492		)	(1,121		)
Allowance for sales returns	(623		)	(731		)
Net trade receivables	128,773			131,922
Other	10,425			12,767
Accounts receivable, net	$	139,198		$	144,689

Allowance for Doubtful Accounts

Changes in the allowance for doubtful accounts were as follows:



(In thousands)	Year Ended December 31,
(In thousands)	2019			2018			2017
Balance at beginning of period	$	1,121		$	1,064		$	904
Additions to costs and expenses	441			305			166
(Write-offs)/Foreign exchange effects	(70		)	(248		)	(6		)
Balance at end of period	$	1,492		$	1,121		$	1,064

Trade receivables associated with these significant customers that totaled more than 10% of our accounts receivable, net were as follows:

⁽¹⁾ Trade receivables associated with this customer did not total more than 10% of our accounts receivable, net for the indicated period.