Old causative formation нийдэх(nijdeh, “to be united”) +-гах(-gah). The change -дг-(-dg-) > -тг-(-tg-) being semi-regular due to -ᠭᠠ-(-ɣa-) suffix having a variant in -ᠬᠠ-(-qa-) after ᠪ(b), ᠳ(d) or ᠰ(s); and -ᠳᠭ-(-dɣ-) clusters being regularly reflected as -тг-(-tg-) in Khalkha.