The constituency names, in this section, follow the format and the spelling used by F. W. S. Craig in his British Parliamentary Election Results series.

The legislation, which was passed in 1536 (using the modern civil year starting on 1 January), was ambiguous as to which communities were enfranchised.

8. c. 11) confirmed that the contributing boroughs could send representatives to take part in the election at the county town.

The original scheme was modified by later legislation and decisions of the House of Commons, which were sometimes made with no regard to precedent or evidence: for example in 1728 it was decided that only the freemen of the borough of Montgomery could participate in the election for that seat, thus disenfranchising the freemen of Llanidloes, Welshpool and Llanfyllin.

The voters in each participating borough cast ballots, which were added together over the whole district to decide the result of the poll.

The enfranchised communities in this district, from 1832, were the six boroughs of Caernarvon, Bangor, Conway, Criccieth, Nevin, and Pwllheli.

In 1918 the constituency was redefined, to include the then local government areas of the Municipal Boroughs of Bangor, Caernarvon, Conway, and Pwllheli; the Urban Districts of Criccieth, Llandudno, Llanfairfechan and Penmaenmawr, as well as the parish of Nevin.

This arrangement was not altered by the redistribution which took effect in February 1974, which was based on the pre-1974 local government boundaries.

The Parliamentary Constituencies (Wales) Order 1995, based on the pre-1996 local authority areas, came into effect in 1997.

It defined this constituency as:- (i) The following wards of the Borough of Arfon, namely, Bethel, Bontnewydd, Cadnant, Deiniolen, Llanberis, Llandwrog, Llanllyfni, Llanrug, Llanwnda, Menai (Caernarfon), Peb-lig, Penisarwaun, Penygroes, Seiont, Talysarn, Waunfawr and Y Felinheli; and (ii) the District of Dwyfor.