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Ducommun Inc./DE – ‘10-Q’ for 9/30/23 – ‘EX-101.DEF’

On:  Wednesday, 11/8/23, at 6:37am ET   ·   For:  9/30/23   ·   Accession #:  1628280-23-37539   ·   File #:  1-08174

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  As Of               Filer                 Filing    For·On·As Docs:Size             Issuer                      Filing Agent

11/08/23  Ducommun Inc./DE                  10-Q        9/30/23   65:6.7M                                   Workiva Inc Wde… FA01/FA

Quarterly Report   —   Form 10-Q

Filing Table of Contents

Document/Exhibit                   Description                      Pages   Size 

 1: 10-Q        Quarterly Report                                    HTML   1.49M 
 2: EX-31.1     Certification -- §302 - SOA'02                      HTML     23K 
 3: EX-31.2     Certification -- §302 - SOA'02                      HTML     23K 
 4: EX-32       Certification -- §906 - SOA'02                      HTML     22K 
10: R1          Cover Page                                          HTML     71K 
11: R2          Condensed Consolidated Balance Sheets               HTML    128K 
12: R3          Condensed Consolidated Balance Sheets               HTML     34K 
                (Parenthetical)                                                  
13: R4          Condensed Consolidated Statements of Income         HTML     92K 
14: R5          Condensed Consolidated Statements of Comprehensive  HTML     46K 
                Income                                                           
15: R6          Condensed Consolidated Statements of Comprehensive  HTML     24K 
                Income (Parenthetical)                                           
16: R7          Condensed Consolidated Statements of Changes in     HTML    110K 
                Shareholders' Equity                                             
17: R8          Condensed Consolidated Statements of Cash Flows     HTML    115K 
18: R9          Summary of Significant Accounting Policies          HTML    122K 
19: R10         Business Combinations                               HTML     44K 
20: R11         Restructuring Activities                            HTML     48K 
21: R12         Inventories                                         HTML     28K 
22: R13         Goodwill                                            HTML     36K 
23: R14         Accrued and Other Liabilities                       HTML     28K 
24: R15         Long-Term Debt                                      HTML     51K 
25: R16         Shareholders? Equity                                HTML     26K 
26: R17         Indemnifications                                    HTML     22K 
27: R18         Income Taxes                                        HTML     28K 
28: R19         Commitments and Contingencies                       HTML     32K 
29: R20         Business Segment Information                        HTML     83K 
30: R21         Summary of Significant Accounting Policies          HTML     71K 
                (Policies)                                                       
31: R22         Summary of Significant Accounting Policies          HTML     99K 
                (Tables)                                                         
32: R23         Business Combinations (Tables)                      HTML     39K 
33: R24         Restructuring Activities (Tables)                   HTML     42K 
34: R25         Inventories (Tables)                                HTML     29K 
35: R26         Goodwill (Tables)                                   HTML     35K 
36: R27         Accrued and Other Liabilities (Tables)              HTML     28K 
37: R28         Long-Term Debt (Tables)                             HTML     37K 
38: R29         Business Segment Information (Tables)               HTML     82K 
39: R30         Summary of Significant Accounting Policies -        HTML     57K 
                Narrative (Details)                                              
40: R31         Summary of Significant Accounting Policies -        HTML     28K 
                Supplemental Cash Flow Items (Details)                           
41: R32         Summary of Significant Accounting Policies -        HTML     56K 
                Weighted Average Number of Shares Outstanding Used               
                to Compute Earnings Per Share (Details)                          
42: R33         Summary of Significant Accounting Policies -        HTML     23K 
                Weighted Average Number of Shares Outstanding                    
                Excluded from Computation of Diluted Earnings                    
                (Details)                                                        
43: R34         Summary of Significant Accounting Policies -        HTML     24K 
                Contract Assets and Liabilities (Details)                        
44: R35         Summary of Significant Accounting Policies -        HTML     44K 
                Disaggregated Revenue (Details)                                  
45: R36         Business Combinations - Narrative (Details)         HTML     92K 
46: R37         Business Combinations - Fair Value of Assets and    HTML     53K 
                Liabilities Acquired (Details)                                   
47: R38         Business Combinations - Estimated Fair Value of     HTML     37K 
                Intangible Assets Acquired (Details)                             
48: R39         Restructuring Activities - Narrative (Details)      HTML     84K 
49: R40         Restructuring Activities - Other Restructuring      HTML     57K 
                Activities (Details)                                             
50: R41         Inventories (Details)                               HTML     28K 
51: R42         Goodwill (Details)                                  HTML     40K 
52: R43         Accrued and Other Liabilities (Details)             HTML     26K 
53: R44         Long-Term Debt - Schedule of Long Term Debt         HTML     45K 
                (Details)                                                        
54: R45         Long-Term Debt - Narrative (Details)                HTML    258K 
55: R46         Shareholders? Equity (Details)                      HTML     37K 
56: R47         Income Taxes (Details)                              HTML     29K 
57: R48         Commitments and Contingencies (Details)             HTML    101K 
58: R49         Business Segment Information - Narrative (Details)  HTML     20K 
59: R50         Business Segment Information - Financial            HTML     50K 
                Information by Reportable Segment (Details)                      
60: R51         Business Segment Information - Segment Assets       HTML     48K 
                (Details)                                                        
63: XML         IDEA XML File -- Filing Summary                      XML    114K 
61: XML         XBRL Instance -- dco-20230930_htm                    XML   1.52M 
62: EXCEL       IDEA Workbook of Financial Report Info              XLSX    147K 
 6: EX-101.CAL  XBRL Calculations -- dco-20230930_cal                XML    170K 
 7: EX-101.DEF  XBRL Definitions -- dco-20230930_def                 XML    515K 
 8: EX-101.LAB  XBRL Labels -- dco-20230930_lab                      XML   1.33M 
 9: EX-101.PRE  XBRL Presentations -- dco-20230930_pre               XML    796K 
 5: EX-101.SCH  XBRL Schema -- dco-20230930                          XSD    134K 
64: JSON        XBRL Instance as JSON Data -- MetaLinks              403±   589K 
65: ZIP         XBRL Zipped Folder -- 0001628280-23-037539-xbrl      Zip    296K 


‘EX-101.DEF’   —   XBRL Definitions — dco-20230930_def


This Exhibit is an XBRL File.


                                                                                                                                                                                
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24 Previous Filings that this Filing References

  As Of               Filer                 Filing    For·On·As Docs:Size             Issuer                      Filing Agent

 6/02/23  Ducommun Inc./DE                  8-K:5,9     5/31/23   11:207K                                   Donnelley … Solutions/FA
 5/04/23  Ducommun Inc./DE                  10-Q        4/01/23   62:6.1M                                   Workiva Inc Wde… FA01/FA
 3/21/23  Ducommun Inc./DE                  8-K:1,7,9   3/20/23   12:854K                                   Donnelley … Solutions/FA
 2/16/23  Ducommun Inc./DE                  10-K       12/31/22  114:14M                                    Workiva Inc Wde… FA01/FA
 8/04/22  Ducommun Inc./DE                  10-Q        7/02/22   65:6.6M                                   Workiva Inc Wde… FA01/FA
 7/18/22  Ducommun Inc./DE                  8-K:1,2,9   7/14/22   11:1.7M                                   Donnelley … Solutions/FA
 4/20/22  Ducommun Inc./DE                  8-K:5,9     4/20/22   11:241K                                   Donnelley … Solutions/FA
 2/23/22  Ducommun Inc./DE                  10-K       12/31/21  111:13M                                    Workiva Inc Wde… FA01/FA
12/20/21  Ducommun Inc./DE                  8-K:1,2,8,912/16/21   12:423K                                   Donnelley … Solutions/FA
12/16/21  Ducommun Inc./DE                  8-K:1,2,7,912/15/21   12:623K                                   Donnelley … Solutions/FA
 7/30/20  Ducommun Inc./DE                  10-Q        6/27/20   71:7M                                     Workiva Inc Wde… FA01/FA
 3/20/20  Ducommun Inc./DE                  DEF 14A     5/06/20    1:4.4M                                   Donnelley … Solutions/FA
 2/20/20  Ducommun Inc./DE                  10-K       12/31/19  120:14M                                    Workiva Inc Wde… FA01/FA
11/26/19  Ducommun Inc./DE                  S-8        11/26/19    4:198K                                   Donnelley … Solutions/FA
10/09/19  Ducommun Inc./DE                  8-K:1,2,7,910/07/19    3:457K                                   Donnelley … Solutions/FA
 5/10/18  Ducommun Inc./DE                  S-8         5/10/18    4:132K                                   Donnelley … Solutions/FA
 3/23/18  Ducommun Inc./DE                  DEF 14A     5/02/18    1:1.1M                                   Donnelley … Solutions/FA
 9/11/17  Ducommun Inc./DE                  8-K:1,2,7,9 9/11/17    3:416K                                   Donnelley … Solutions/FA
 3/06/17  Ducommun Inc./DE                  10-K       12/31/16  113:13M                                    Workiva Inc Wde… FA01/FA
 1/27/17  Ducommun Inc./DE                  8-K:1,9     1/23/17    3:86K                                    Donnelley … Solutions/FA
 1/09/17  Ducommun Inc./DE                  8-K:5,9     1/03/17    3:46K                                    Donnelley … Solutions/FA
 2/22/10  Ducommun Inc./DE                  10-K       12/31/09    9:1.1M                                   Donnelley … Solutions/FA
 3/04/04  Ducommun Inc./DE                  10-K       12/31/03   11:953K                                   Donnelley … Solutions/FA
 3/04/99  Ducommun Inc./DE                  10-K405    12/31/98    9:115K                                   Bowne - Biv/FA
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