A cross is a geometrical figure consisting of two lines or bars perpendicular to each other, dividing one or two of the lines in half. The lines usually run vertically and horizontally; if they run diagonally, the design is technically termed a saltire (https://en.wikipedia.org/wiki/Saltire)

The word cross was introduced to English in the 10th century as the term for the instrument of the torturous execution of Christ, ultimately from Latin crux, via Old Irish cross. The word can nowadays refer to the geometrical shape unrelated to its Christian significance. The Latin Crux (translating Greek stauros) was a Roman torture device used for crucifixion.

https://www.holylandtreasuresonline.com/Store/Content/ResourceArticle/-3/1/21.html

The cross as a symbol dates back to a long time period of paganism before Christ and the cross. The sideways cross with the shorter end toward the left is representative of the Cross used by Denmark on their flag. Rumor says that is the oldest national flag and is referred to as the Nordic cross. The use of the sideways cross in that manner is wide spread on flags including state flags. 

It is also an old math symbol. And, at one time, it represented the picking up of your cross and following. No need to be upright, since the Christian has no need of being crucified as an individual. That task according to the Bible has been completed. 

There are allusions to it being an anchor hoisted on the bow indicated at full sail, in which case the direction of the cross has importance or which side the short end is on or the side that drops into the water when the anchor is set. 

Satanism jumps in on the symbolism too. There was way too much to read on that. Personally for much of jewelry business it is an economical way to manufacture a clasp and use it as a marketing tool as well. A long time ago the short end slid through an oblong circle fastening a chain together becoming a link.

https://hubpages.com/style/forum/273686/what-is-the-meaning-of-the-sideways-cross-necklace-that-everyone-seems-to-be-wea

"I read on some thread that the sideways cross means humanity, earth, and in between heaven and hell.  If you ask me, it's just a trend fueled by celbs, as usual."

"I think originally it was just to be a fashion statement, but personally I find it offensive.  When I see a sideways cross, I think of someone who has fallen from Grace.  However, I love seeing someone wear their cross as I feel the person wearing it is just proud to be a Christian, but I don't think God intended for us to bring attention to ourselves by making them sideways or gaudy."

https://hubpages.com/style/forum/273686/what-is-the-meaning-of-the-sideways-cross-necklace-that-everyone-seems-to-be-wea