When completed in 1969, the building at 122 Leadenhall Street was tall with 14 storeys above and three storeys under ground. It was originally designed as a pair with the Commercial Union headquarters (now called St. Helen's) by the architects Gollins Melvin Ward Partnership. The two buildings have a central compressional concrete core and have suspended floors which hang using the steel 'chords' visible on the exterior of the building, which are hung from power trusses at the top of the building (and in the case of No. 1 Undershaft, a further central power-truss). It is an example of a tension structure; at the time, it was considered one of the most complex glass-fronted buildings in the United Kingdom. The architect acknowledged the influence of Mies van der Rohe.