Digital Realty is a company involved in datacenter acquisition, ownership, development and operation as well as providing colocation services. Digital Realty's customers include domestic and international companies across multiple industry verticals ranging from information technology and Internet enterprises, to manufacturing and financial services. Digital Realty's 101 properties, excluding three properties held as investments in unconsolidated joint ventures, comprise approximately 18.3 million square feet as of December 31, 2011 including 2.4 million square feet of space held for redevelopment. Digital Realty does its business in 31 markets throughout Europe, North America, Singapore and Australia. Digital Realty operates as a publicly traded Real Estate Investment Trust (REIT) on the New York Stock Exchange under the ticker symbol "DLR". The company currently employs approximately 950 people throughout the United States, Europe, and Asia Pacific. Corporate headquarters for the company is located in San Francisco, California. The company also maintains regional offices in Los Angeles, Phoenix, Dallas, Chicago, Boston, New York, Northern Virginia, London, Paris, Dublin, Singapore and Sydney. Digital Realty claims to have attained additional notability through development of a number of innovative efficiency metrics such as breaking down stages of data center construction into discrete units, utilizing its POD Architecture, measured in terms of kW of IT load, and "Power Usage Effectiveness" (PUE) which is the ratio of power delivered to IT equipment to the total amount of power used by the data center facility. The company is a member of The Green Grid and has helped pioneer concepts of energy efficient and energy conserving data center design.