The route passes through the heart of Wales, and is also known as cy|'''Lôn Las Cymru'''|Wales' green lane. It is largely north–south from Holyhead to Cardiff or Chepstow, and in total measures some 400km (200miles) in length. Some of its route follows the trackbed of former railway lines, such as Welsh: [[Lôn Las Menai]], Welsh: [[Lôn Eifion]], the Welsh: Mawddach|italic=no|nocat=y Trail and the Taff Trail; in other places, the route is on public highways.
Both ends are easily accessible via the rail network.
This largely follows the Taff Trail:
Cardiff | Pontypridd | Abercynon | Merthyr Tydfil | Brecon
Lon Las Cymru provides an alternative south route for those coming into Wales from the Severn Bridge, following Route 42 from Chepstow to Glasbury, where it joins Route 8:
Chepstow | Usk | Abergavenny | Glasbury
There is also a small loop at the top of Route 42 providing links to Hay-on-Wye.
Brecon | Talgarth | Glasbury | Erwood | Builth Wells | Newbridge on Wye | Llanwrthwl | Rhayader | Llangurig | Llanidloes | Staylittle | Machynlleth
Machynlleth | Dolgellau | Trawsfynydd | Penrhyndeudraeth | Porthmadog
Porthmadog | Penygroes | Caernarfon | Bangor | Holyhead