Malamute asked the question about NCO chevrons on US army NCOs from 1830 – 50, which prompted me to do some research on the topic
Digging up a reference from Gregory Urwin was very helpful
Chevrons were introduced for NCO ranks in the 1821 regulations. In the infantry, they were white worsted chevrons, with a sergeant wearing a single chevron, point upward, on each sleeve below the elbow – a corporal wore a single chevron on the right upper sleeve, again point upward
In 1825 the regs moved the sergeant's chevron to above the elbow, while corporals wore a chevron on both sleeves, but below the elbow
In 1832 chevrons were removed from dress uniforms, sergeant's rank being indicated by worsted epaulettes, lace loops on the cuffs, a white stripe down the trousers and a red sash for the First Sergeant
At the the time of the Seminole War chevrons were worn above the elbow, but thye indicated rank, one white chevron (point upwards) for each five years of service – in 1833 the US Dragoon introduced downward pointing chevrons on the upper arm for rank – two for a corporal, three for a sergeant – which the rest of the army adopted in imitation
The 1847 regulations introduced upward pointing chevrons for rank, worn above the elbow, again two chevrons for corporals, three for sergeants – an upward-pointing chevron in dark blue on the lower sleeve was given for five years of service, with red borders for service in time of war. At the time of the Mexican-American war, some of the volunteer regiments used downward pointing chevrons (I have a picture of a sergeant from a South Carolina regiment with what look to be white downward pointing chevrons in Mexico)
The 1851 regulations specified downward pointing chevrons in branch colour for rank, above the elbow, two for corporals, three for sergeants, with upward pointing half-chevrons on the lower sleeve for five years of service, in branch colour with red borders for service in time of war – this was what was used during the Civil War
Malamute – hope this helps