REVENUE

Sales includes revenue from contracts with customers from roof and foundation ventilation products; centralized mail systems and electronic package solutions; rain dispersion products and roofing accessories; expanded and perforated metal; perimeter security solutions; expansion joints and structural bearings; designing, engineering, manufacturing and installation of solar racking systems and greenhouse structures.

Revenue recognition

Revenue is recognized when, or as, the Company transfers control of promised products or service to a customer in an amount that reflects the consideration the Company expects to be entitled in exchange for transferring those products or service. Refer to Note 16 of this quarterly report on Form 10-Q for additional information related to revenue recognized by timing of transfer of control by reportable segment.

As of March 31, 2019, the Company's remaining performance obligations are part of contracts that have an original expected duration of one year or less.

Contract assets and contract liabilities

Contract assets consist of costs in excess of billings. Contract liabilities consist of billings in excess of cost and unearned revenue. Unearned revenue relates to payments received in advance of performance under the contract and is recognized when the Company performs under the contract. Unearned revenue is presented within accrued expenses in the Company's consolidated balance sheet.

The following table presents the beginning and ending balances and significant changes in the costs in excess of billings and billings in excess of cost balance during the three months ended March 31, 2019 and 2018, respectively (in thousands):



	March 31, 2019			December 31, 2018
Costs in excess of billings	$	25,519		$	22,634
Billings in excess of cost	(18,259		)	(17,857		)
Unearned revenue	(12,917		)	(12,028		)



	Three Months Ended March 31, 2019		Three Months Ended March 31, 2018
Revenue recognized in the period from:
Amounts included in billings in excess of cost at the beginning of the period	$	9,697	$	8,340
Amounts included in unearned revenue at the beginning of the period	$	4,661	$	1,836