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Urban vestiges that serve absolutely no purpose have a name. They're called "Thomassons". Vanessa Berry with bricked up windows on Hickson Road. There are even steel bars to protect one of the bricked up doorways. Credit: Steven Siewert

An old barge station under Sydney Harbour Bridge. The term "Thomasson" was coined by Japanese artist Genpei Akasegawa. In 1972 Akasegawa came across a lone staircase that had no door at the top. But then he noticed something even stranger: the railing had recently been fixed. Credit: Steven Siewert

The incomplete Maldon to Dombarton rail bridge near Picton. Why Thomasson? It comes from the professional baseball player Gary Thomasson, signed by Japan’s Yomiuri Giants for a vast amount of money but who then spent his last two seasons in 1981-82 near to setting a league strikeout record before being benched. Credit: Nick Moir

Unused steps from nowhere to nowhere along The Hungry Mile, Hickson Road, The Rocks. Artist Genpei considered Thomasson’s redundant team position as a fitting analogy for “an object, part of a building, that was maintained in good condition, but with no purpose, to the point of becoming a work of art”. Credit: Steven Siewert

There are some 20 different categories of Thomasson. The Devouring Tree is defined as a tree that absorbs part of a fence or wire while still growing, such as this one in Clemton Park. Credit: Steven Siewert

Then there's the Useless Doorway, such as this elevated one to nowhere at Callan Park. Credit: Steven Siewert

The Atago is an object sticking out at the side of the road, with no clear purpose, such as this bollard in Summer Hill. Credit: Steven Siewert

The Uyama is a sign or hoarding with letters missing, such as this example at Callan Park. Credit: Steven Siewert

The Boundary is described as a guardrail, fence or wall whose purpose is not immediately clear, such as this old fence with useless signage at Old Coal Loader, Waverton. Credit: Steven Siewert

The Outie is a protuberance from a sealed up wall, such as a door knob or tap. This old pipe is not connected to anything and protrudes from a wall along Hickson Road, The Rocks. Credit: Steven Siewert

Disused public toilets at Macquarie Place Park near Circular Quay. Credit: Steven Siewert

A horse water trough in Botany. Credit: Steven Siewert

An old bridge near White Bay. Credit: Steven Siewert

Sections of the old abandoned Princes Highway near Waterfall. Credit: Nick Moir

The ferry between Lugarno and Illawong closed in 1974, but the ferry ramp remains where the ferry would slide onto shore for people and vehicles to board. Credit: Nick Moir

A mysterious building opposite Pier 1. Credit: Steven Siewert

An old bank depository drop-off box in Sydney. Credit: Steven Siewert

A mystery small brick wall in Greenwich. Credit: Steven Siewert

A small building in Callan Park, which looks like an outdoor toilet. Credit: Steven Siewert

An old piece of machinery on a wall at Old Coal Loader, Waverton. Credit: Steven Siewert

An old wharf with stairs leading into the water at Old Coal Loader, Waverton. Credit: Steven Siewert

The incomplete Maldon to Dombarton rail bridge near Picton. Credit: Nick Moir

The ferry between Lugarno and Illawong closed in 1974, but the ferry ramp remains where the ferry would slide onto shore for people and vehicles to board. Credit: Nick Moir

The "Pansy" was a steam train line that travelled between Richmond and Kurrajong in Sydney's north west. A bridge crossing Redbank Creek near North Richmond is one of only a few surviving reminders of the rail line. Credit: Nick Moir

Sections of the old abandoned Princes Highway near Waterfall. Credit: Nick Moir