Cloud & Cognitive Software
* Recast to reflect segment changes.
** 2019 results were impacted by Red Hat purchase accounting.
Cloud & Cognitive Software revenue of $23,200 million increased 4.5 percent as reported (6 percent adjusted for currency) in 2019 compared to the prior year. There was strong growth in Cloud & Data Platforms, as reported and at constant currency, driven primarily by the acquisition of Red Hat in the third quarter of 2019. Red Hat had continued strong performance since the acquisition, in Red Hat Enterprise Linux (RHEL), application development and emerging technologies, led by OpenShift and Ansible. Red Hat and IBM are driving synergies with strong adoption of Cloud Paks since their introduction, expansion of our combined client base and more than 2,000 clients using our hybrid cloud platform. Cognitive Applications also grew as reported and at constant currency. Transaction Processing Platforms declined year to year as reported, but grew 1 percent adjusted for currency driven by strong fourth-quarter performance.
Cognitive Applications revenue of $5,765 million grew 2.3 percent as reported (4 percent adjusted for currency) compared to the prior year, driven by double-digit growth as reported and adjusted for currency in Security, and growth in industry verticals such as IoT. The Security performance included continued strong results in threat management software and services offerings. Within IoT, we had good revenue performance across the portfolio as we continued to invest in new offerings and industry-specific solutions.
Cloud & Data Platforms revenue of $9,499 million increased 10.4 percent as reported (12 percent adjusted for currency) compared to the prior year. Performance was driven by the addition of RHEL and OpenShift and the continued execution of the combined Red Hat and IBM hybrid strategy.
Transaction Processing Platforms revenue of $7,936 million decreased 0.5 percent as reported, but grew 1 percent adjusted for currency in 2019, compared to the prior year. Revenue performance reflects the ongoing investment in IBM platforms, and the timing of larger transactions that are tied to client business volumes and buying cycles.
Within Cloud & Cognitive Software, cloud revenue of $4.2 billion grew 40 percent as reported and 42 percent adjusted for currency year to year, reflecting the acquisition of Red Hat and client adoption of our hybrid cloud offerings.
Table:
($ in millions) |  |  |  | 
For the year ended December 31: | 2019 | 2018 | Yr.-to-Yr. Percent Change ** | Yr.-to-Yr. Percent Change Adjusted for Currency **
Cloud & Cognitive Software external revenue | $23,200 | $22,209 | 4.5% | 6.2%
Cognitive Applications | $ 5,765 | $ 5,633 | 2.3% | 3.9%
Cloud & Data Platforms | 9,499 | 8,603 | 10.4 | 12.3
Transaction Processing Platforms | 7,936 | 7,974 | (0.5) | 1.4


Question: What caused the increase in Cloud & Cognitive Software revenue in 2019?
Answer: There was strong growth in Cloud & Data Platforms, as reported and at constant currency, driven primarily by the acquisition of Red Hat in the third quarter of 2019. Red Hat had continued strong performance since the acquisition, in Red Hat Enterprise Linux (RHEL), application development and emerging technologies, led by OpenShift and Ansible.