Zepler doubling

A kind of doubling in which one piece moves along a line allowing a second to move onto the same line behind it; the first piece then moves again in the same direction as before.

Three different forms are recognized:

1. (Ordinary) Zepler doubling. Two pieces are of different strength and the stronger one first makes an anti-critical move (and subsequently the weaker one comes behind).
2. Loyd-Zepler doubling. Two pieces are of different strength and the weaker one first makes an anti-critical move (and subsequently the stronger one comes behind).
3. Brunner-Zepler doubling. Two pieces are of same strength.

See also: Loyd-Zepler doubling; Brunner-Zepler doubling; Turton doubling.