Carn Fiaclach
Carn Fiaclach, a 749m Simm near Loch Rannoch 

A Simm is a hill in Britain that is at least 600 metres high and has a drop of at least 30 metres on all sides. The term Simm is derived from Six-hundred Metre Mountain or Monteto, where monteto is the word for hill in Esperanto. The second M may therefore stand for mountain, mam, meall, mullach, moel or mynydd according to the nature and location of the hill. The Simms have a simple definition but a complex history. They were introduced by Alan Dawson in 2010 as a way of unifying several of his own published lists that he had come to regard as unnecessarily complicated and outdated:

Most of these booklets are now out of print, though copies of the last two are still available and include useful notes about numerous hills. The Simms have not as yet been published as single unified list. More information about the Simms is available in these sources:

By the end of October 2022 just five people had climbed all 2531 Simms in Britain: Ken Whyte in 2010, Iain Thow in 2015, Michael Earnshaw in 2019, Rob Woodall in 2021 and Anne Bunn in 2022. Michael climbed them all within eighteen years and was the first person to have climbed all the Marilyns and all the Simms, with Rob Woodall the second. In July 2023 Alan Dawson, Eric Young and Graham Illing all climbed their final Simm.

Subsimms are British hills that are at least 600 metres high and have a drop of 20-29.9m drop on all sides. By July 2023 a total of 699 Subsimms had been identified. No-one has yet bagged them all, though Ken Whyte, Alan Dawson, Alan Whatley and Iain Thow have climbed over 600.