In the "Western Tradition" of Necromancy, the Triangle is favored in Conjurations. What does this symbol REALLY mean? Is it to ensnare the Spirit? No. Does it represent the Fiery Will of the Necromancer? No. What it actually represents is the Avatar. It simply serves as a homing beacon for the Spirit. Like a GPS. This Avatar is activated with an object link that belongs to the Deceased together with a medium for manifestation, such as purified water, blood, incense etc.
The question arises: Do you need a Triangle of Manifestation in Necromancy? No. Why Not? Simply put, many cultures has Conjured Spirits of the Dead successfully without using Western Systems or symmetrical symbols. Does this indicate that the Western Tradition of Necromancy is flawed? No, humanity is flawed, not the Great Arte.
In order to conduct a successful Necromantic ceremony you will need Spiritual Authority, Association and Expertise. Without this you will either fail or become victim of a Malicious Spirit. And people, these Spirits will RUIN your life to such an extent that no god or devil will be able to remove them, save only an Agent of Death, Master Necromancer or perhaps an Exorcist. Dabble in Necromancy, we double dare you.
Necromancy is the most volatile Occult practice in existence. Why? Because the Dead were once as we are now and some of these Spirits will do ANYTHING to reside within a living body again. Unfortunately there are A LOT of weak minded and spiritually timid individuals out there that makes easy targets for even the weakest of human Dead. You may envision these ravenous Spirits as legions upon legions of shaded masses hovering around us, probing and prodding the very Being of the living, desperately trying to latch on to a potential victim. There are many braggarts and loud mouths that underestimate the Dead but these individuals do not know that the Dead are very much capable to devour even Daemonic Beings, as they are ravenous unending - The Malicious Dead is not known as the "Locust Horde of Lote' for their timid nature.
Ancestry workings - as in working with deceased Ancestors may or may not be considered as being part of Necromancy in the traditional sense. This is a subject matter which borders between Religion and Occult. This is a tricky subject with no right or wrong to it. This paradigm's functionality can balance on a knife's edge for practical usefulness on a day to day level. Consider this. The Dead will be the same as they were living, unless they manage to spiritually ascend (which is rare while dead - not impossible though) so if you had a grandfather that was a vagabond or druggie, then obviously he will have those same traits as a Spirit. So, you can either choose to honor your ancestor by means of offerings, prayers, conversations etc or make use of them as working Spirits. But strongly consider the nature and personality of your ancestor if you choose the latter. This can mean the difference between usefulness and well...grief. At the end of the day it simply depends on your preference as an individual, you may or may not choose to work with your ancestors.
In order to master any craft you need to master yourself first. By doing this you remove wanting aspects and flaws within your Being, thus building upon what remains in order to gain spiritual power and authority. But...the choice is yours.