4 lb. cucumbers (about 6 large), cut in 1/4-inch slices
3 medium onions, cut into 1/8-inch slices
3 c. sugar
3 c. cider vinegar
4 tsp. canning/pickling salt
1 1/2 tsp. mustard seed
1/2 tsp. alum

Combine cucumbers and onions. In a large bowl, combine remaining ingredients, stirring until sugar is dissolved. Pour over cukes and onions mixture; mix well. Cover and chill overnight. May be refrigerated up to 2 weeks.

(From Taste of Home Magazine.)